[DAGCombiner] Add more rules to fold shuffles.

This patch adds two new rules to the DAGCombiner:
 1.  shuffle (shuffle A, Undef, M0), B, M1 -> shuffle A, B, M2
 2.  shuffle (shuffle A, Undef, M0), A, M1 -> shuffle A, Undef, M2

We only do this if the combined shuffle is legal for the target.

Example:
;;
define <4 x float> @test(<4 x float> %a, <4 x float> %b) {
  %1 = shufflevector <4 x float> %a, <4 x float> undef, <4 x i32><i32 6, i32 0, i32 1, i32 7>
  %2 = shufflevector <4 x float> %1, <4 x float> %b, <4 x i32><i32 1, i32 2, i32 4, i32 5>
  ret <4 x i32> %2
}
;;

(using llc -mcpu=corei7 -march=x86-64)
Before, the x86 backend generated:
  pshufd $120, %xmm0, %xmm0
  shufps $-108, %xmm0, %xmm1
  movaps %xmm1, %xmm0

Now the x86 backend generates:
  movsd %xmm1, %xmm0