[InstSimplify] fix/improve folding with an SNaN operand

There are 2 issues here:

1. In the default LLVM FP environment (regular FP math instructions),
   SNaN is some flavor of "don't care" which we will nail down in
   D143074, so this is just a quality-of-implementation improvement
   for default FP.
2. In the constrained FP environment (constrained intrinsics), SNaN
   must not propagate through a math operation; it has to be quieted
   according to IEEE-754 spec. That is independent of exception
   handling mode, so the current behavior is a miscompile.

Differential Revision: https://reviews.llvm.org/D143505