[DAG] Further improve the logic in DAGCombiner that folds a pair of shuffles into a single shuffle if the resulting mask is legal.

This patch teaches the DAGCombiner how to fold shuffles according to the
following new rules:
  1. shuffle(shuffle(x, y), undef) -> x
  2. shuffle(shuffle(x, y), undef) -> y
  3. shuffle(shuffle(x, y), undef) -> shuffle(x, undef)
  4. shuffle(shuffle(x, y), undef) -> shuffle(y, undef)

The backend avoids to combine shuffles according to rules 3. and 4. if
the resulting shuffle does not have a legal mask. This is to avoid introducing
illegal shuffles that are potentially expanded into a sub-optimal sequence of
target specific dag nodes during vector legalization.

Added test case combine-vec-shuffle-2.ll to verify that we correctly triggers
the new rules when combining shuffles.