[RELOAD] Don't assume subreg mem address is ok

[RELOAD] Don't assume subreg mem address is ok

This patch fixes a case where reload blindly assumes a subreg mem is
OK if its address has been partially reloaded by legitimize_reload_address.

	PR rtl-optimization/72771
	* reload.c (find_reloads): Don't assume that a subreg mem is OK
	when find_reloads_toplev returns address_reloaded==-1.
	(alternative_allows_const_pool_ref): Update comment.
testsuite/
	* gcc.c-torture/compile/pr72771.c: New.

From-SVN: r239549