[DAGCombiner] Improve the shuffle-vector folding logic.

Canonicalize shuffles according to rules:
 *  shuffle(A, shuffle(A, B)) -> shuffle(shuffle(A,B), A)
 *  shuffle(B, shuffle(A, B)) -> shuffle(shuffle(A,B), B)
 *  shuffle(B, shuffle(A, Undef)) -> shuffle(shuffle(A, Undef), B)

This patch helps identifying more shuffle pairs that could be combined reusing
the already existing rules in the DAGCombiner.

Added new test 'combine-vec-shuffle-5.ll' to verify that the canonicalized
shuffles are now folded into a single shuffle node by the DAGCombiner.
Added more test cases to 'combine-vec-shuffle-4.ll'.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213504 91177308-0d34-0410-b5e6-96231b3b80d8