[DAGCombiner] Improved target independent vector shuffle combine rule.

This patch improves the existing algorithm in DAGCombiner that
attempts to fold shuffles according to rule:
  shuffle(shuffle(x, y, M1), undef, M2) -> shuffle(y, undef, M3)

Before this change, there were cases where the DAGCombiner conservatively
avoided folding shuffles even if the resulting mask would have been legal.
That is because the algorithm wrongly assumed that commuting
an illegal shuffle mask would always produce an illegal mask.

With this change, we now correctly compute the commuted shuffle mask before
calling method 'isShuffleMaskLegal' on it.
On X86, this improves for example the codegen for the following function:

define <4 x i32> @test(<4 x i32> %A, <4 x i32> %B) {
  %1 = shufflevector <4 x i32> %B, <4 x i32> %A, <4 x i32> <i32 1, i32 2, i32 6, i32 7>
  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 2, i32 3, i32 2, i32 3>
  ret <4 x i32> %2
}

Before this change the X86 backend (-mcpu=corei7) generated
the following assembly code for function @test:
  shufps $-23, %xmm0, %xmm1  # xmm1 = xmm1[1,2],xmm0[2,3]
  movhlps %xmm1, %xmm1       # xmm1 = xmm1[1,1]
  movaps %xmm1, %xmm0

Now we produce:
  movhlps %xmm0, %xmm0       # xmm0 = xmm0[1,1]

Added extra test cases in combine-vec-shuffle-2.ll to verify that we correctly
fold according to the above-mentioned rule.