[x86, SSE] AVX1 PR28129 (256-bit all-ones rematerialization)

Further perf tests on Jaguar indicate that:

vxorps  %ymm0, %ymm0, %ymm0
vcmpps  $15, %ymm0, %ymm0, %ymm0

is consistently faster (by about 9%) than:

vpcmpeqd  %xmm0, %xmm0, %xmm0
vinsertf128  $1, %xmm0, %ymm0, %ymm0

Testing equivalent code on a SandyBridge (E5-2640) puts it slightly (~3%) faster as well.

Committed on behalf of @dtemirbulatov

Differential Revision: https://reviews.llvm.org/D32416

llvm-svn: 302989