[CGP, x86] add tests for potential memcmp expansion; NFC

No IR tests were added with rL304313 ( https://reviews.llvm.org/D28637 ),
so I want these for extra coverage if we enable memcmp expansion for x86.
As shown, nothing is expanded for x86 in CGP yet.

Also fundamentally, we're doing an IR transform, so we should have IR tests 
for just that part. If something goes wrong, we need to know if the bug is
in CGP or later lowering.

llvm-svn: 305011