[x86, MemCmpExpansion] allow 2 pairs of loads per block (PR33325)

This is the last step needed to fix PR33325:
https://bugs.llvm.org/show_bug.cgi?id=33325

We're trading branch and compares for loads and logic ops. 
This makes the code smaller and hopefully faster in most cases.

The 24-byte test shows an interesting construct: we load the trailing scalar 
elements into vector registers and generate the same pcmpeq+movmsk code that 
we expected for a pair of full vector elements (see the 32- and 64-byte tests).

Differential Revision: https://reviews.llvm.org/D41714

llvm-svn: 321934