re PR debug/49522 (Divide by zero in validate_subreg in emit-rtl.c:695)

re PR debug/49522 (Divide by zero in validate_subreg in emit-rtl.c:695)

	PR debug/49522
	* df-problems.c (dead_debug_reset): Remove dead_debug_uses
	referencing debug insns that have been reset.
	(dead_debug_insert_before): Don't assert reg is non-NULL,
	instead return immediately if it is NULL.

	* gcc.dg/debug/pr49522.c: New test.

M    gcc/ChangeLog
A    gcc/testsuite/gcc.dg/debug/pr49522.c
M    gcc/testsuite/ChangeLog
M    gcc/df-problems.c

From-SVN: r175987