[X86] Perform VSELECT DAG combines also before DAG type legalization.

If the DAG already has only legal types, then the second round of DAG combines
is skipped. In this case VSELECT+SETCC patterns that match a more efficient
instruction (e.g. min/max) are never recognized.

This fix allows VSELECT+SETCC combines if the types are already legal before DAG
type legalization.

Reviewer: Nadav
llvm-svn: 190105