[FPEnv][InstSimplify] Constrained FP support for NaN

Currently InstructionSimplify.cpp knows how to simplify floating point
instructions that have a NaN operand. It does not know how to handle the
matching constrained FP intrinsic.

This patch teaches it how to simplify so long as the exception handling
is not "fpexcept.strict".

Differential Revision: https://reviews.llvm.org/D103169