[DebugInfo] MachineSink: Insert undef DBG_VALUEs when sinking instructions

When we sink DBG_VALUEs between blocks, we simply move the DBG_VALUE
instruction to below the sunk instruction. However, we should also mark
the variable as being undef at the original location, to terminate any
earlier variable location. This patch does that -- plus, if the
instruction being sunk is a copy, it attempts to propagate the copy
through the DBG_VALUE, replacing the destination with the source.

Differential Revision: https://reviews.llvm.org/D58238