[X86] Fix crash on assert of non-simple type after type-legalization

The function combineShuffleToVectorExtend in DAGCombine might generate an illegal typed node after "legalize types" phase, causing assertion on non-simple type to fail afterwards.

Adding a type check in case the combine is running after the type legalize pass.

Differential Revision: https://reviews.llvm.org/D37330

llvm-svn: 312438