[x86] fold vector (X > -1) & Y to shift+andn (2nd try)

[x86] fold vector (X > -1) & Y to shift+andn (2nd try)

The first try at this patch ( bf5748a1af0d ) was reverted ( 5be64d416481 )
because it could crash. The cause of that problem was failing to account
for the optional peek-through-bitcast in the enclosing function.

This version of the patch adds a clause to avoid the fold in case of
bitcasts because it is unlikely to be profitable in that scenario.

A test case based on https://llvm.org/PR52504 was added to make sure
we don't have that problem again.

Original commit message:

and (pcmpgt X, -1), Y --> pandn (vsrai X, BitWidth-1), Y

This avoids the -1 constant vector in favor of an arithmetic shift
instruction if it exists (the ISA is still not complete after all
these years...).

We catch this pattern late in combining by matching PCMPGT, so it
should not interfere with more general folds.

Differential Revision: https://reviews.llvm.org/D113603