[RegisterCoalescer] Move debug value after rematerialize trivial def

Summary:
The associated debug value is updated when the virtual source register
of a copy is completely eliminated and replaced with a rematerialize
value in the defed register of the copy. As the debug value now is
associated with another register it also need to be moved, otherwise
the debug value isn't valid.

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: MatzeB, llvm-commits, qcolombet

Differential Revision: https://reviews.llvm.org/D38024

llvm-svn: 317880