[SelectionDAG] Move RegsForValue into SelectionDAGBuilder.h. NFC.

Summary:
The exported class will be used in later change, in
StatepointLowering.cpp.  It is still internal to SelectionDAG (not
exported via include/).

Reviewers: reames, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9478

llvm-svn: 236554