YJIT: Fallback megamorphic opt_case_dispatch

[k0kubun]

opt_case_dispatch_megamorphic accounts for 68% of exits on rubocop. This PR fixes it by falling back to compile subsequent instructions as is after 20 chains.

This speeds up rubocop by 3%.

before: ruby 3.4.0dev (2024-02-08T23:52:45Z master e2aa00ca66) +YJIT [x86_64-linux]
after: ruby 3.4.0dev (2024-02-08T23:53:29Z yjit-opt-case 2193dc2f13) +YJIT [x86_64-linux]

-------  -----------  ----------  ----------  ----------  -------------  ------------
bench    before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
rubocop  132.1        4.7         128.0       4.5         0.94           1.03
-------  -----------  ----------  ----------  ----------  -------------  ------------

This also speeds up optcarrot --opt by 17%, which people sometimes use when they compare their JIT against YJIT.

# before
ruby -v --yjit bin/optcarrot-bench --opt
ruby 3.4.0dev (2024-02-08T23:52:45Z master e2aa00ca66) +YJIT [x86_64-linux]
fps: 164.09038383764974

# after
$ ruby -v --yjit bin/optcarrot-bench --opt
ruby 3.4.0dev (2024-02-08T23:53:29Z yjit-opt-case 2193dc2f13) +YJIT [x86_64-linux]
fps: 192.09196646229304

[maximecb]

Nice work, and thanks for making sure that we deliver on our optcarrot goals! 😆