[x86] allow pairs of PCMPEQ for vector-sized integer equality comparisons (PR33325)

This is an extension of D31156 with the goal that we'll allow memcmp() == 0 expansion 
for x86 to use 2 pairs of loads per block.

The memcmp expansion pass (formerly part of CGP) will generate this kind of pattern 
with oversized integer compares, so we want to transform these into x86-specific vector
nodes before legalization splits things into scalar chunks.

See PR33325 for more details:
https://bugs.llvm.org/show_bug.cgi?id=33325

Differential Revision: https://reviews.llvm.org/D41618

llvm-svn: 321656