[DAGCombiner] Fold truncate(build_vector(x,y)) -> build_vector(truncate(x),truncate(y))

If scalar truncates are free, attempt to pre-truncate build_vectors source operands.

Only attempt to do this before legalization as we often end up with truncations/extensions during build_vector lowering.

Differential Revision: https://reviews.llvm.org/D59654

llvm-svn: 357161