middle-end/118801 - excessive redundant DEBUG BEGIN_STMT

middle-end/118801 - excessive redundant DEBUG BEGIN_STMT

The following addresses the fact that we keep an excessive amount of
redundant DEBUG BEGIN_STMTs - in the testcase it sums up to 99.999%
of all stmts, sucking up compile-time in IL walks.  The patch amends
the GIMPLE DCE code that elides redundant DEBUG BIND stmts, also
pruning uninterrupted sequences of DEBUG BEGIN_STMTs, keeping only
the last of each set of DEBUG BEGIN_STMT with unique location.

	PR middle-end/118801
	* tree-ssa-dce.cc (eliminate_unnecessary_stmts): Prune
	sequences of uninterrupted DEBUG BEGIN_STMTs, keeping only
	the last of a set with unique location.