[MachineSink][DebugInfo] Correctly sink DBG_VALUEs

As reported in PR38952, postra-machine-sink relies on DBG_VALUE insns being
adjacent to the def of the register that they reference. This is not always
true, leading to register copies being sunk but not the associated DBG_VALUEs,
which gives the debugger a bad variable location.

This patch collects DBG_VALUEs as we walk through a BB looking for copies to
sink, then passes them down to performSink. Compile-time impact should be
negligable.

Differential Revision: https://reviews.llvm.org/D53992

llvm-svn: 345996