operand scanner TLC

This applies some TLC to mark_address_taken which ends up setting
TREE_ADDRESSABLE on nodes where it doesn't have any semantics.
It also does work (incomplete) that get_base_address already does,
likewise we'll never get WITH_SIZE_EXPR in this context and thus
get_base_address never fails.

2021-05-18  Richard Biener  <rguenther@suse.de>

	* tree-ssa-operands.c (mark_address_taken): Simplify.