fold-const: Optimize &"foo"[0] == "foo" [PR89074]

fold-const: Optimize &"foo"[0] == "foo" [PR89074]

This is a non-C++ related part from the PR89074 address_compare changes.
For "foo" == "foo" we already optimize this from the (cmp @0 @0)
simplification, because we use operand_equal_p in that case
and operand_equal_p also compares the STRING_CSTs bytes rather than
just addresses.

2022-01-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/89074
	* fold-const.cc (address_compare): Consider different STRING_CSTs
	with the same lengths that memcmp the same as equal, not different.

	* gcc.dg/tree-ssa/pr89074.c: New test.