re PR tree-optimization/31602 (Overflow warning causes GDB -Werror build failure)

re PR tree-optimization/31602 (Overflow warning causes GDB -Werror build failure)

./:	PR tree-optimization/31602
	* tree-ssa-loop-ch.c (copy_loop_headers): Set TREE_NO_WARNING for
	conditionals in the copied loop header.
	* tree-cfg.c (fold_cond_expr_cond): Don't issue undefined overflow
	warnings if TREE_NO_WARNING is set.
	* doc/invoke.texi (Warning Options): Clarify that
	-Wstrict-overflow does not warn about loops.
testsuite/:
	PR tree-optimization/31602
	* gcc.dg/Wstrict-overflow-11.c: We no longer issue a warning.

From-SVN: r124120