tree-optimization/99728 - improve LIM for loops with aggregate copies

tree-optimization/99728 - improve LIM for loops with aggregate copies

This improves LIM by recording aggregate copies for disambiguation
purposes instead of as UNANALYZABLE_MEM which will prevent any
invariant or store motion across it.  This allows four of the six
references in the loop of the testcase to be promoted.

2021-07-07  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99728
	* tree-ssa-loop-im.c (gather_mem_refs_stmt): Record
	aggregate copies.
	(mem_refs_may_alias_p): Add assert we handled aggregate
	copies elsewhere.
	(sm_seq_valid_bb): Give up when running into aggregate copies.
	(ref_indep_loop_p): Handle aggregate copies as never
	being invariant themselves but allow other refs to be
	disambiguated against them.
	(can_sm_ref_p): Do not try to apply store-motion to aggregate
	copies.

	* g++.dg/opt/pr99728.C: New testcase.