[X86] Fold cmpeq/ne(trunc(logic(x)),0) --> cmpeq/ne(logic(x),0)

Fixes the issues noted in PR48768, where the and/or/xor instruction had been promoted to avoid i8/i16 partial-dependencies, but the test against zero had not.

We can almost certainly relax this fold to work for any truncation, although it breaks a number of existing folds (notable movmsk folds which tend to rely on the truncate to determine the demanded bits/elts in the source vector).

There is a reverse combine in TargetLowering.SimplifySetCC so we must wait until after legalization before attempting this.