[WebAssembly] Expand vector frem instructions (#174854)
Commit
https://github.com/llvm/llvm-project/commit/6ad41bcc498b7c186ceb54616c3697fc9873c04a
changed how frem is expanded during legalization and it
broke WebAssembly but we were missing test coverage. We want to maintain
our previous behavior of unrolling vectors and using a libcall to
implement scalar frem. I'm not sure why this now has to be different
(in ISelLowering) from other libcalls like fsin which work the same way
in the end, but this code does accurately describe what we want.
Fixes: https://github.com/emscripten-core/emscripten/issues/25991
