[x86] add tests for memcmp expansion; NFC

We already had a test to demonstrate PR33325:
https://bugs.llvm.org/show_bug.cgi?id=33325

I'm adding tests for general memcmp expansion (see D34005 / D33963) and:
https://bugs.llvm.org/show_bug.cgi?id=33329

...plus non-power-of-2 sizes, so we can see what that looks like currently or if expanded.

llvm-svn: 304979