gimple-fold: Fix ICE in maybe_canonicalize_mem_ref_addr on debug stmt [PR96354]

gimple-fold: Fix ICE in maybe_canonicalize_mem_ref_addr on debug stmt [PR96354]

In debug stmts, we are less strict about what is and what is not accepted
there, so this patch just punts on optimization of a debug stmt rather than
ICEing.

2020-08-04  Jakub Jelinek  <jakub@redhat.com>

	PR debug/96354
	* gimple-fold.c (maybe_canonicalize_mem_ref_addr): Add IS_DEBUG
	argument.  Return false instead of gcc_unreachable if it is true and
	get_addr_base_and_unit_offset returns NULL.
	(fold_stmt_1) <case GIMPLE_DEBUG>: Adjust caller.

	* g++.dg/opt/pr96354.C: New test.