[SelectionDAG] Refactor code by adding RegsForValue::getRegsAndSizes(). NFCI

Summary:
Added a helper method in RegsForValue to get a list with
all the <RegNumber, RegSize> pairs that we want to iterate
over in SelectionDAGBuilder::EmitFuncArgumentDbgValue and
in SelectionDAGBuilder::visitIntrinsicCall.

Reviewers: vsk

Reviewed By: vsk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46360

llvm-svn: 331510