[VE] VECustomDAG builder class

VECustomDAG's functions simplify emitting VE custom ISD nodes. The class
is just a stub now. We add more functions, in particular for the
VP->VVP->VE lowering, to VECustomDAG as we build up vector isel.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D116103