[SelectionDAG] Make an argument optional in RFV::getCopyToRegs. NFC.

Summary:
We default the value argument to nullptr.  The only use of the value is
in diagnosePossiblyInvalidConstraint and that seems to be resilient to
it being nullptr.

Reviewers: atrick, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9479

llvm-svn: 236555