middle-end/101478 - ICE with degenerate address during gimplification

When we gimplify &MEM[0B + 4] we are re-folding the address in case
types are not canonical which ends up with a constant address that
recompute_tree_invariant_for_addr_expr ICEs on.  Properly guard
that call.

	PR middle-end/101478
	* gimplify.cc (gimplify_addr_expr): Check we still have an
	ADDR_EXPR before calling recompute_tree_invariant_for_addr_expr.

	* gcc.dg/pr101478.c: New testcase.