[DAGCombiner] Use tryFoldToZero to simplify some code and make it work...

[DAGCombiner] Use tryFoldToZero to simplify some code and make it work correctly between LegalTypes and LegalOperations.

The original code avoided creating a zero vector after type legalization, but if we're after type legalization the type we have is legal. The real hazard we need to avoid is creating a build vector after op legalization. tryFoldToZero takes care of checking for this.

llvm-svn: 346119