Avoid marking TARGET_MEM_REF bases addressable

Avoid marking TARGET_MEM_REF bases addressable

The following does no longer mark TARGET_MEM_REF bases addressable,
mimicing MEM_REFs beahvior here.  In contrast to the latter,
TARGET_MEM_REF RTL expansion expects to always operate on memory
though, so make sure we expand them so.

2021-05-19  Richard Biener  <rguenther@suse.de>

	* cfgexpand.c (discover_nonconstant_array_refs_r): Make
	sure TARGET_MEM_REF bases are expanded as memory.
	* tree-ssa-operands.c (operands_scanner::get_tmr_operands):
	Do not mark TARGET_MEM_REF bases addressable.
	* tree-ssa.c (non_rewritable_mem_ref_base): Handle
	TARGET_MEM_REF bases as never rewritable.
	* gimple-walk.c (walk_stmt_load_store_addr_ops): Do not
	walk TARGET_MEM_REF bases as address-takens.
	* tree-ssa-dce.c (ref_may_be_aliased): Handle TARGET_MEM_REF.