[FPEnv][InstSimplify] Fold fsub -0.0, -X ==> X

Currently the fsub optimizations in InstSimplify don't know how to fold
-0.0 - (-X) to X when the constrained intrinsics are used. This adds partial
support. The rest of the support will come later with work on the IR
matchers.

This review is split out from D107285.

Differential Revision: https://reviews.llvm.org/D123396