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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@236555 91177308-0d34-0410-b5e6-96231b3b80d8

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