PR tree-optimization/91570 - ICE in get_range_strlen_dynamic on a conditional
of two strings

gcc/Changelog:
	* tree-ssa-strlen.c (get_range_strlen_dynamic): Handle null and
	non-constant minlen, maxlen and maxbound.

gcc/testsuite/Changelog:
	* gcc.dg/pr91570.c: New test.

From-SVN: r276105