[DAGCombiner] Fix a crash caused by a missing check for legal type when trying to fold shuffles.

Verify that DAGCombiner does not crash when trying to fold a pair of shuffles
according to rule (added at r212539):
  (shuffle (shuffle A, Undef, M0), Undef, M1) -> (shuffle A, Undef, M2)

The DAGCombiner avoids folding shuffles if the resulting shuffle dag node
is not legal for the target. That means, the resulting shuffle must have
legal type and legal mask.

Before, the DAGCombiner only called method
'TargetLowering::isShuffleMaskLegal' to check if it was "safe" to fold according
to the above-mentioned rule. However, this caused a crash in the x86 backend
since method 'isShuffleMaskLegal' always expects to be called on a
legal vector type.