[SFN] stabilize find_bb_boundaries

[SFN] stabilize find_bb_boundaries

If find_bb_boundaries is given a block with zero or one nondebug insn
beside debug insns, it shouldn't purge dead edges, because without
debug insns we wouldn't purge them at that point.  Doing so may change
the order in which edges are processed, and ultimately lead to
different transformations to the CFG and then to different
optimizations.

We shouldn't, however, retain debug insns after control flow insns, so
if we find debug insns after a single insn that happens to be a
control flow insn, do the debug insn cleanups, but still refrain from
purging dead edges at that point.

for  gcc/ChangeLog

	* cfgbuild.c (find_bb_boundaries): Don't purge dead edges if,
	without debug insns, we wouldn't, but clean up debug insns
	after a control flow insn nevertheless.

From-SVN: r255567