Debug info: If the RegisterCoalescer::reMaterializeTrivialDef() is
eliminating all uses of a vreg, update any DBG_VALUE describing that vreg
to point to the rematerialized register instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223401 91177308-0d34-0410-b5e6-96231b3b80d8