[llvm] [clang] Revert "InstCombine: Fold is.fpclass(x, fcInf) to fabs+fcmp" (PR #76338)

[Matt Arsenault via cfe-commits]

arsenm wrote:

I still think fabs+fcmp is a better canonical form. Between some targets offering free fabs, and other combines between fabs and sources/sinks, I think it's better to split out this way. Any codegen regression should be addressed in the backend.

https://github.com/llvm/llvm-project/pull/76338