X86: Enable SSE memory intrinsics even when stack alignment is less than 16 bytes.

The stack realignment code was fixed to work when there is stack realignment and
a dynamic alloca is present so this shouldn't cause correctness issues anymore.

Note that this also enables generation of AVX instructions for memset
under the assumptions:
- Unaligned loads/stores are always fast on CPUs supporting AVX
- AVX is not slower than SSE
We may need some tweaked heuristics if one of those assumptions turns out not to
be true.

Effectively reverts r58317. Part of PR2962.

llvm-svn: 167967