[libclc] [libclc] Optimize isfpclass-like CLC builtins (PR #124145)

[Matt Arsenault via cfe-commits]

https://github.com/arsenm approved this pull request.

I remember debugging inconsistent behavior on different platforms in treatment of signaling nans and denormal flushing with these queries. I think these days we emit them as is.fpclass anyway, and then instcombine turns them into fcmp when valid. This may be a backwards system, so this may need revisiting in the future 

https://github.com/llvm/llvm-project/pull/124145