[clang] [llvm] InstCombine: Use SimplifyDemandedFPClass on fmul (PR #177490)

[Yingwei Zheng via cfe-commits]

dtcxzyw wrote:

Compile-time impact:
+0.05% on llvm-opt-benchmark
+0.06% on ctmark: https://llvm-compile-time-tracker.com/compare.php?from=572dabdbf637e006f666fd069fe1aebf151c7a5f&to=c333f19668d6ab95e4c4225d6c3d5575face165e&stat=instructions:u

https://github.com/llvm/llvm-project/pull/177490