[x86] use SSE/AVX ops for non-zero memsets (PR27100)

Move the memset check down to the CPU-with-slow-SSE-unaligned-memops case: this allows fast
targets to take advantage of SSE/AVX instructions and prevents slow targets from stepping
into a codegen sinkhole while trying to splat a byte into an XMM reg.

Follow-on bugs exposed by the current codegen are:
https://llvm.org/bugs/show_bug.cgi?id=27141
https://llvm.org/bugs/show_bug.cgi?id=27143

Differential Revision: http://reviews.llvm.org/D18566

llvm-svn: 265029