re PR tree-optimization/87859 (store-merging miscompilation of mesa)

re PR tree-optimization/87859 (store-merging miscompilation of mesa)

	PR tree-optimization/87859
	* gimple-ssa-store-merging.c (struct merged_store_group): Add
	only_constants and first_nonmergeable_order members.
	(merged_store_group::merged_store_group): Initialize them.
	(merged_store_group::do_merge): Clear only_constants member if
	adding something other than INTEGER_CST store.
	(imm_store_chain_info::coalesce_immediate_stores): Don't merge
	stores with order >= first_nonmergeable_order.  Use
	merged_store->only_constants instead of always recomputing it.
	Set merged_store->first_nonmergeable_order if we've skipped any
	stores.  Attempt to merge overlapping INTEGER_CST stores that
	we would otherwise skip.

	* gcc.dg/store_merging_24.c: New test.
	* gcc.dg/store_merging_25.c: New test.

From-SVN: r265794