[ARM] Generate VDUP(Const) from constant buildvectors

If we cannot otherwise use a VMOVimm/VMOVFPimm/VMVNimm, fall back to
producing a VDUP(const) as opposed to a constant pool load. This will at
least be smaller codesize and can allow the VDUP to be folded into other
instructions.

Differential Revision: https://reviews.llvm.org/D103808