[X86] Don't combine (x86cmp (trunc (movmsk (bitcast X))), 0) if the truncate discards...

[X86] Don't combine (x86cmp (trunc (movmsk (bitcast X))), 0) if the truncate discards unknown bits.

We have transform that tries turn a pmovmskb into movmskps/pd or
movmskps to movmskpd. This transform isn't valid if the truncate
discarded bits that might be set by the original movmsk.

We could fix this by inserting an AND after the new movmsk to discard
the equivalent of the truncated bits, but I've left that for later
patch.

Fixes PR52567.

Differential Revision: https://reviews.llvm.org/D114306