[DAGCombiner] Improve the folding of target independet shuffles to Undef.

When combining a pair of shuffle nodes, check if the combined shuffle mask is
trivially Undef. In case, immediately fold that pair of shuffles to Undef.

The lack of checks for undef masks was the root-cause of a poor-codegen bug
in the dag combiner.

Example:
  %1 = shufflevector <4 x i32> %A, <4 x i32> %B, <4 x i32> <i32 4, i32 1, i32 1, i32 6>
  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 0, i32 4, i32 1, i32 6>
  %3 = shufflevector <4 x i32> %2, <4 x i32> undef, <4 x i32> <i32 1, i32 5, i32 3, i32 3>

Before this patch, on x86 (with -mcpu=corei7) we failed to fold the entire
sequence to Undef value and therefore we generated:
  shufps $-123, %xmm1, $xmm0
  pshufd $-46, %xmm0, %xmm0

With this patch, the entire shuffle sequence is folded to Undef and no
shuffles are generated in the output assembly.

Added new test cases to test 'combine-vec-shuffle-5.ll'.