PR tree-optimization/91996 - fold non-constant strlen relational expressions

gcc/testsuite/ChangeLog:

	PR tree-optimization/91996
	* gcc.dg/strlenopt-80.c: New test.
	* gcc.dg/strlenopt-81.c: New test.

gcc/ChangeLog:

	PR tree-optimization/91996
	* tree-ssa-strlen.c (maybe_warn_pointless_strcmp): Improve location
	information.
	(compare_nonzero_chars): Add an overload.
	(count_nonzero_bytes): Add an argument.  Call overload above.
	Handle non-constant lengths in some range.
	(handle_store): Add an argument.
	(check_and_optimize_stmt): Pass an argument to handle_store.

From-SVN: r277076