Propagate DBG_VALUE entries when there are unvisited predecessors

Variables are sometimes missing their debug location information in
blocks in which the variables should be available. This would occur
when one or more predecessor blocks had not yet been visited by the
routine which propagated the information from predecessor blocks.

This is addressed by only considering predecessor blocks which have
already been visited.

The solution to this problem was suggested by Daniel Berlin on the
LLVM developer mailing list.

Differential Revision: https://reviews.llvm.org/D24927

llvm-svn: 282506