[DAGCombiner] Don't reduce BUILD_VECTOR to BITCAST before LegalizeTypes if VT is legal.

Targets may lose some optimization opportunities for certain vector operation
if we reduce BUILD_VECTOR to BITCAST early.

And if VT is not legal, reduce BUILD_VECTOR to BITCAST before LegailizeTypes
can get benefit. Because type-legalizer often scalarizes illegal type of vectors.

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D156645