[X86] Remove incorrect use of known bits in shuffle simplification.

This reverts commit 2a419a0b.

The result of a shufflevector must not propagate poison from any element
other than the one noted in the shuffle mask.

The regressions outside of fptoui-may-overflow.ll can probably be
recovered some other way; for example, using isGuaranteedNotToBePoison.

See discussion on https://reviews.llvm.org/D106053 for more background.

Differential Revision: https://reviews.llvm.org/D106222