[DAG] Add BuildVectorSDNode::getConstantRawBits helper

We have several places where we need to extract the raw bits data from a BUILD_VECTOR node, so consolidate this to a single helper function that handles Undefs and Integer/FP constants, including implicit truncation.

This should make it easier to extend D113202 to handle more constant folding of bitcasted constant data.

Differential Revision: https://reviews.llvm.org/D113351