[X86] Improve buildFromShuffleMostly for AVX

For a 256-bit BUILD_VECTOR consisting mostly of shuffles of 256-bit vectors,
both the BUILD_VECTOR and its operands may need to be legalized in multiple
steps.  Consider:

(v8f32 (BUILD_VECTOR (extract_vector_elt (v8f32 %vreg0,) Constant<1>),
                     (extract_vector_elt %vreg0, Constant<2>),
                     (extract_vector_elt %vreg0, Constant<3>),
                     (extract_vector_elt %vreg0, Constant<4>),
                     (extract_vector_elt %vreg0, Constant<5>),
                     (extract_vector_elt %vreg0, Constant<6>),
                     (extract_vector_elt %vreg0, Constant<7>),
                     %vreg1))

a. We can't build a 256-bit vector efficiently so, we need to split it into
two 128-bit vecs and combine them with VINSERTX128.

b. Operands like (extract_vector_elt (v8f32 %vreg0), Constant<7>) needs to be
split into a VEXTRACTX128 and a further extract_vector_elt from the
resulting 128-bit vector.

c. The extract_vector_elt from b. is lowered into a shuffle to the first
element and a movss.

Depending on the order in which we legalize the BUILD_VECTOR and its
operands[1], buildFromShuffleMostly may be faced with:

(v4f32 (BUILD_VECTOR (extract_vector_elt
                      (vector_shuffle<1,u,u,u> (extract_subvector %vreg0, Constant<4>), undef),
                      Constant<0>),
                     (extract_vector_elt
                      (vector_shuffle<2,u,u,u> (extract_subvector %vreg0, Constant<4>), undef),
                      Constant<0>),
                     (extract_vector_elt
                      (vector_shuffle<3,u,u,u> (extract_subvector %vreg0, Constant<4>), undef),
                      Constant<0>),
                     %vreg1))

In order to figure out the underlying vector and their identity we need to see
through the shuffles.

[1] Note that the order in which operations and their operands are legalized is
only guaranteed in the first iteration of LegalizeDAG.

Fixes <rdar://problem/16296956>