[x86, InstCombine] transform x86 AVX masked loads to LLVM intrinsics

The intended effect of this patch in conjunction with:
http://reviews.llvm.org/rL259392
http://reviews.llvm.org/rL260145

is that customers using the AVX intrinsics in C will benefit from combines when
the load mask is constant:

__m128 mload_zeros(float *f) {
  return _mm_maskload_ps(f, _mm_set1_epi32(0));
}

__m128 mload_fakeones(float *f) {
  return _mm_maskload_ps(f, _mm_set1_epi32(1));
}

__m128 mload_ones(float *f) {
  return _mm_maskload_ps(f, _mm_set1_epi32(0x80000000));
}

__m128 mload_oneset(float *f) {
  return _mm_maskload_ps(f, _mm_set_epi32(0x80000000, 0, 0, 0));
}

...so none of the above will actually generate a masked load for optimized code.

This is the masked load counterpart to:
http://reviews.llvm.org/rL262064

llvm-svn: 262269