codegen: Remove last use-cases of Variant::has

[Hendiadyoin1]

We always know the internal typeid, so we can do this lookup our selves
and put less strain on the c++-compiler

[ilyapopov]

This PR cuts 0.25s (2%):

hyperfine -r 5 "cd jakt/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp" "cd jakt-pr1042/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp"
Benchmark 1: cd jakt/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp
  Time (mean ± σ):     14.880 s ±  0.154 s    [User: 14.275 s, System: 0.585 s]
  Range (min … max):   14.736 s … 15.123 s    5 runs
 
Benchmark 2: cd jakt-pr1042/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp
  Time (mean ± σ):     14.634 s ±  0.048 s    [User: 14.174 s, System: 0.446 s]
  Range (min … max):   14.588 s … 14.705 s    5 runs
 
Summary
  'cd jakt-pr1042/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp' ran
    1.02 ± 0.01 times faster than 'cd jakt/build/ && clang++ -std=c++20 -I ../runtime/ -Wno-parentheses-equality -Wno-trigraphs -Wno-user-defined-literals jakt.cpp'