X86 target specific DAG combine: turn build_vector (load x), (load x+4),

X86 target specific DAG combine: turn build_vector (load x), (load x+4),
(load x+8), (load x+12), <0, 1, 2, 3> to a single 128-bit load (aligned and
unaligned).

e.g.

__m128 test(float a, float b, float c, float d) {
  return _mm_set_ps(d, c, b, a);
}

_test:
        movups 4(%esp), %xmm0
        ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29042 91177308-0d34-0410-b5e6-96231b3b80d8