[DebugInfo] MachineSink: find more DBG_VALUEs to sink

In the Pre-RA machine sinker, previously we were relying on all DBG_VALUEs
being immediately after the instruction that defined their operands. This
isn't a valid assumption, as a variable location change doesn't
necessarily correspond to where the value is computed. In this patch, we
collect DBG_VALUEs that might need sinking as we walk through a block,
and sink all of them if their defining instruction is sunk.

This patch adds some copy propagation too, so that if we sink a copy inst,
the now non-dominated paths can use the copy source for the variable
location.

Differential Revision: https://reviews.llvm.org/D58386