[InstSimplify] propagate poison through FP ops

We already have this fold:
  fadd float poison, 1.0 --> poison
...via ConstantFolding, so this makes the behavior consistent
if the other operand(s) are non-constant.

The fold for undef was added before poison existed as a
value/type in IR.

This came up in D102673 / D103169
because we're trying to sort out the more complicated handling
for constrained math ops.
We should have the handling for the regular instructions done
first, so we can build on that (or diverge as needed).

Differential Revision: https://reviews.llvm.org/D104383