Previous logic did not handle the case where the result bit size was
between 32 and 64 bits inclusive. I updated the if-statements for more
precise handling.
An alternative solution would have been to abort FastISel in case the
result type is not legal for FastISel.
Resolves: #64222.
This PR began as an investigation into the root cause of
https://github.com/ziglang/zig/issues/20966.
Godbolt link showing incorrect codegen on 20.1.0:
https://godbolt.org/z/cEr4vY7d4.
9d89b05f11