[DebugInfo] Avoid dropping location info across block boundaries

LiveDebugValues propagates variable locations between blocks by creating
new DBG_VALUE insts in the successors, then interpreting them when it
passes back through the block at a later time. However, this flushes out
any extra information about the location that LiveDebugValues holds: for
example, connections between variable locations such as discussed in
D65368. And as reported in PR42772 this causes us to lose track of the
fact that a spill-location is actually a spill, not a register location.

This patch fixes that by deferring the creation of propagated DBG_VALUEs
until after propagation has completed: instead location propagation occurs
only by sharing location ID numbers between blocks. 

Differential Revision: https://reviews.llvm.org/D66412

llvm-svn: 369508