[x86] avoid intermediate splat for non-zero memsets (PR27100)

Follow-up to http://reviews.llvm.org/D18566 and http://reviews.llvm.org/D18676 -
where we noticed that an intermediate splat was being generated for memsets of
non-zero chars.

That was because we told getMemsetStores() to use a 32-bit vector element type,
and it happily obliged by producing that constant using an integer multiply.

The 16-byte test that was added in D18566 is now equivalent for AVX1 and AVX2
(no splats, just a vector load), but we have PR27141 to track that splat difference.

Note that the SSE1 path is not changed in this patch. That can be a follow-up.
This patch should resolve PR27100.

llvm-svn: 265161