Fixed tryFoldToZero() for vector types that need expansion.

Summary:
Moved the requirement for SelectionDAG::getConstant() to return legally
typed nodes slightly earlier. There were two optional DAGCombine passes
that were missed out and were required to produce type-legal DAGs.

Simplified a code-path in tryFoldToZero() to use SelectionDAG::getConstant().
This provides support for both promoted and expanded vector types whereas the
previous code only supported promoted vector types.

Fixes a "Type for zero vector elements is not legal" assertion detected by
an llvm-stress generated test.

Reviewers: resistor

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2251