PR tree-optimization/86622 - incorrect strlen of array of array plus variable offset

PR tree-optimization/86622 - incorrect strlen of array of array plus variable offset
PR tree-optimization/86532 - Wrong code due to a wrong strlen folding starting with r262522

gcc/ChangeLog:

	PR tree-optimization/86622
	PR tree-optimization/86532
	* builtins.h (string_length): Declare.
	* builtins.c (c_strlen): Correct handling of non-constant offsets.
	(check_access): Be prepared for non-constant length ranges.
	(string_length): Make extern.
	* expr.c (string_constant): Only handle the minor non-constant
	array index.  Use string_constant to compute the length of
	a generic string constant.

gcc/testsuite/ChangeLog:

	PR tree-optimization/86622
	PR tree-optimization/86532
	* gcc.c-torture/execute/strlen-2.c: New test.
	* gcc.c-torture/execute/strlen-3.c: New test.
	* gcc.c-torture/execute/strlen-4.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@262958 138bc75d-0d04-0410-961f-82ee72b054a4