[InstCombine] Don't add fcmp instructions to strictfp functions (#81498)

The strictfp attribute has the requirement that "LLVM will not introduce
any new floating-point instructions that may trap". The llvm.is.fpclass
intrinsic is documented as "The function never raises floating-point
exceptions", and the fcmp instruction may raise one, so we can't
transform the former into the latter in functions with the strictfp
attribute.