[x86] add an SSE2 + fast-unaligned accesses run for memset nonzero tests

Was there really no other way to splat a byte in SSE2?
    punpcklbw {{.*#+}} xmm0 = xmm0[0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7]
    pshuflw {{.*#+}} xmm0 = xmm0[0,0,0,0,4,5,6,7]
    pshufd {{.*#+}} xmm0 = xmm0[0,0,1,1]

llvm-svn: 265172