[DebugInfo] Correctly update debug users of SSA values in tail duplication

During tail duplication, SSA values may be updated and have their uses
replaced with a virtual register, and any debug instructions that use
that value are deleted. This patch fixes the implementation of the debug
instruction deletion to work correctly for debug instructions that use
the SSA value multiple times, by batching deletions so that we don't
attempt to delete the same instruction twice.

Differential Revision: https://reviews.llvm.org/D106557