[InstCombine] Fold `fpto{s|u}i non-norm` to zero (#85569)

This patch enables more optimization after canonicalizing `fmul X, 0.0`
into a copysign.
I decide to implement this fold in InstCombine because
`computeKnownFPClass` may be expensive.

Alive2: https://alive2.llvm.org/ce/z/ASM8tQ