When gimplifying an address operand we may expose an indirect
ref via DECL_VALUE_EXPR for example.  This is dealt with in the
code already but it fails to consider that INDIRECT_REFs get
gimplified to MEM_REFs.

Fixed which makes the ICE observed on x86_64-netbsd go away.

2020-04-07  Richard Biener  <rguenther@suse.de>

	PR middle-end/94479
	* gimplify.c (gimplify_addr_expr): Also consider generated
	MEM_REFs.

	* gcc.dg/torture/pr94479.c: New testcase.