[DAGCombiner] Add more rules to combine shuffle vector dag nodes.

This patch teaches the DAGCombiner how to fold a pair of shuffles
according to rules:
  1.  shuffle(shuffle A, B, M0), B, M1) -> shuffle(A, B, M2)
  2.  shuffle(shuffle A, B, M0), A, M1) -> shuffle(A, B, M3)

The new rules would only trigger if the resulting shuffle has legal type and
legal mask.

Added test 'combine-vec-shuffle-3.ll' to verify that DAGCombiner correctly
folds shuffles on x86 when the resulting mask is legal. Also added some negative
cases to verify that we avoid introducing illegal shuffles.