Reduce the number of branches in jit_exec

[k0kubun]

The primary purpose of this is the readability of jit_exec. I confirmed that this doesn't slow down both JITs.

Benchmark

YJIT

before: ruby 3.2.0dev (2022-11-13T22:41:08Z master e377875cff) +YJIT [x86_64-linux]
after: ruby 3.2.0dev (2022-11-13T22:48:13Z jit-exec f809f4effd) +YJIT [x86_64-linux]

----------  -----------  ----------  ----------  ----------  ------------  -------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  before/after  after 1st itr
railsbench  1113.8       1.7         1070.3      1.8         1.04          1.01
----------  -----------  ----------  ----------  ----------  ------------  -------------

$ ./run_benchmarks.rb railsbench -e "before::/opt/rubies/before/bin/ruby --yjit" -e "after::/opt/rubies/after/bin/ruby --yjit"
Running benchmark "railsbench" (1/1)
setarch x86_64 -R taskset -c 7 /opt/rubies/before/bin/ruby --yjit -I ./harness benchmarks/railsbench/benchmark.rb
ruby 3.2.0dev (2022-11-13T22:41:08Z master e377875cff) +YJIT [x86_64-linux]
Command: bundle check 2> /dev/null || bundle install
The Gemfile's dependencies are satisfied
Command: bin/rails db:migrate db:seed
Deleted all 100 posts
Creating 100 posts....................................................................................................
itr #1: 1233ms
itr #2: 1053ms
itr #3: 1075ms
itr #4: 1110ms
itr #5: 1086ms
itr #6: 1076ms
itr #7: 1077ms
itr #8: 1076ms
itr #9: 1126ms
itr #10: 1112ms
itr #11: 1109ms
itr #12: 1117ms
itr #13: 1084ms
itr #14: 1144ms
itr #15: 1079ms
itr #16: 1127ms
itr #17: 1092ms
itr #18: 1121ms
itr #19: 1124ms
itr #20: 1128ms
itr #21: 1113ms
itr #22: 1110ms
itr #23: 1147ms
itr #24: 1091ms
itr #25: 1081ms
Average of last 10, non-warmup iters: 1113ms
Running benchmark "railsbench" (1/1)
setarch x86_64 -R taskset -c 7 /opt/rubies/after/bin/ruby --yjit -I ./harness benchmarks/railsbench/benchmark.rb
ruby 3.2.0dev (2022-11-13T22:48:13Z jit-exec f809f4effd) +YJIT [x86_64-linux]
Command: bundle check 2> /dev/null || bundle install
The Gemfile's dependencies are satisfied
Command: bin/rails db:migrate db:seed
Deleted all 100 posts
Creating 100 posts....................................................................................................
itr #1: 1217ms
itr #2: 1073ms
itr #3: 1109ms
itr #4: 1109ms
itr #5: 1110ms
itr #6: 1109ms
itr #7: 1080ms
itr #8: 1104ms
itr #9: 1108ms
itr #10: 1039ms
itr #11: 1046ms
itr #12: 1070ms
itr #13: 1068ms
itr #14: 1078ms
itr #15: 1104ms
itr #16: 1074ms
itr #17: 1070ms
itr #18: 1072ms
itr #19: 1086ms
itr #20: 1073ms
itr #21: 1069ms
itr #22: 1045ms
itr #23: 1033ms
itr #24: 1070ms
itr #25: 1108ms
Average of last 10, non-warmup iters: 1070ms
Total time spent benchmarking: 58s

end_time: 2022-11-13 14:50:26 PST (-0800)
before: ruby 3.2.0dev (2022-11-13T22:41:08Z master e377875cff) +YJIT [x86_64-linux]
after: ruby 3.2.0dev (2022-11-13T22:48:13Z jit-exec f809f4effd) +YJIT [x86_64-linux]

----------  -----------  ----------  ----------  ----------  ------------  -------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  before/after  after 1st itr
railsbench  1113.8       1.7         1070.3      1.8         1.04          1.01
----------  -----------  ----------  ----------  ----------  ------------  -------------
Legend:
- before/after: ratio of before/after time. Higher is better for after. Above 1 represents a speedup.
- after 1st itr: ratio of before/after time for the first benchmarking iteration.

MJIT

before: ruby 3.2.0dev (2022-11-13T22:41:08Z master e377875cff) +MJIT [x86_64-linux]
after: ruby 3.2.0dev (2022-11-13T22:48:13Z jit-exec f809f4effd) +MJIT [x86_64-linux]

---------  -----------  ----------  ----------  ----------  ------------  -------------
bench      before (ms)  stddev (%)  after (ms)  stddev (%)  before/after  after 1st itr
optcarrot  1467.7       1.4         1448.2      1.0         1.01          1.00
---------  -----------  ----------  ----------  ----------  ------------  -------------

Running benchmark "optcarrot" (1/1)
setarch x86_64 -R taskset -c 7 /opt/rubies/before/bin/ruby --mjit-max-cache\=100 -I ./harness benchmarks/optcarrot/benchmark.rb
ruby 3.2.0dev (2022-11-13T22:41:08Z master e377875cff) +MJIT [x86_64-linux]
itr #1: 4406ms
itr #2: 2782ms
itr #3: 2858ms
itr #4: 2718ms
itr #5: 2447ms
itr #6: 2421ms
itr #7: 2374ms
itr #8: 2402ms
itr #9: 2446ms
itr #10: 2927ms
itr #11: 3010ms
itr #12: 3034ms
itr #13: 2477ms
itr #14: 1481ms
itr #15: 1483ms
itr #16: 1476ms
itr #17: 1478ms
itr #18: 1484ms
itr #19: 1480ms
itr #20: 1436ms
itr #21: 1451ms
itr #22: 1440ms
itr #23: 1448ms
itr #24: 1491ms
itr #25: 1488ms
Average of last 10, non-warmup iters: 1467ms
Running benchmark "optcarrot" (1/1)
setarch x86_64 -R taskset -c 7 /opt/rubies/after/bin/ruby --mjit-max-cache\=100 -I ./harness benchmarks/optcarrot/benchmark.rb
ruby 3.2.0dev (2022-11-13T22:48:13Z jit-exec f809f4effd) +MJIT [x86_64-linux]
itr #1: 4390ms
itr #2: 2788ms
itr #3: 2852ms
itr #4: 2688ms
itr #5: 2513ms
itr #6: 2475ms
itr #7: 2368ms
itr #8: 2462ms
itr #9: 2424ms
itr #10: 2901ms
itr #11: 3010ms
itr #12: 2995ms
itr #13: 2454ms
itr #14: 1466ms
itr #15: 1461ms
itr #16: 1465ms
itr #17: 1460ms
itr #18: 1455ms
itr #19: 1463ms
itr #20: 1427ms
itr #21: 1430ms
itr #22: 1430ms
itr #23: 1433ms
itr #24: 1454ms
itr #25: 1461ms
Average of last 10, non-warmup iters: 1448ms
Total time spent benchmarking: 109s

end_time: 2022-11-13 14:55:12 PST (-0800)
before: ruby 3.2.0dev (2022-11-13T22:41:08Z master e377875cff) +MJIT [x86_64-linux]
after: ruby 3.2.0dev (2022-11-13T22:48:13Z jit-exec f809f4effd) +MJIT [x86_64-linux]

---------  -----------  ----------  ----------  ----------  ------------  -------------
bench      before (ms)  stddev (%)  after (ms)  stddev (%)  before/after  after 1st itr
optcarrot  1467.7       1.4         1448.2      1.0         1.01          1.00
---------  -----------  ----------  ----------  ----------  ------------  -------------
Legend:
- before/after: ratio of before/after time. Higher is better for after. Above 1 represents a speedup.
- after 1st itr: ratio of before/after time for the first benchmarking iteration.