[x86] fix miscompile from wrongly identified fneg

We may need to peek through a bitcast when identifying an fneg idiom
via its pool constant, but we can't allow a different-sized constant
in that match.

This is noted in issue #55758 with an example that needs fast-math,
but as the test here shows, this has potential to miscompile more
generally (no fast-math required).

Differential Revision: https://reviews.llvm.org/D126775