re PR tree-optimization/84503 (store-merging miscompilation on powerpc64 with -O3...

re PR tree-optimization/84503 (store-merging miscompilation on powerpc64 with -O3 since r241789)

	PR tree-optimization/84503
	* gimple-ssa-store-merging.c (merged_store_group::merge_into): Compute
	width as info->bitpos + info->bitsize - start.
	(merged_store_group::merge_overlapping): Simplify width computation.
	(check_no_overlap): New function.
	(imm_store_chain_info::try_coalesce_bswap): Compute expected
	start + width and last_order of the group, fail if check_no_overlap
	fails.
	(imm_store_chain_info::coalesce_immediate_stores): Don't merge info
	to group if check_no_overlap fails.

	* gcc.dg/pr84503-1.c: New test.
	* gcc.dg/pr84503-2.c: New test.

From-SVN: r257891