[DAGCombiner] Enable tryToFoldExtendOfConstant to run after legalize vector ops

It should be ok to create a new build_vector after legal operations so long as it doesn't cause an infinite loop in DAG combiner.

Unfortunately, X86's custom constant folding in combineVSZext is hiding any test changes from this. But I'm trying to get to a point where that X86 specific code isn't necessary at all.

Differential Revision: https://reviews.llvm.org/D54285

llvm-svn: 346728