[CodeGen][ExpandMemcmp] Add an option for allowing overlapping loads.

Summary:
This allows expanding {7,11,13,14,15,21,22,23,25,26,27,28,29,30,31}-byte memcmp
in just two loads on X86. These were previously calling memcmp.

Reviewers: spatel, gchatelet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55263

llvm-svn: 349731