InstCombine: Introduce SimplifyDemandedUseFPClass

This is the floating-point analog of SimplifyDemandedBits. If we know
the edge cases are assumed impossible in uses, it's possible to prune
upstream edge case handling.

Start by only using this on returns in functions with nofpclass
returns (where I'm surprised there are no other combines), but this
can be extended to include any other nofpclass use or FPMathOperator
with flags.

Partially addresses issue #64870

https://reviews.llvm.org/D158648