[DebugInfo][InstrRef] Fix error in copy handling in InstrRefLDV

Currently, an error exists when InstrRefBasedLDV observes transfers of
variables across copies, which causes it to lose track of variables
under certain circumstances, resulting in shorter lifetimes for those
variables as LDV gives up searching for live locations for them. This
patch fixes this issue by storing the currently tracked values in
the destination first, then updating them manually later without
clobbering or assigning them the wrong value.

Differential Revision: https://reviews.llvm.org/D128101