re PR tree-optimization/78821 (GCC7: Copying whole 32 bits structure field by field...

re PR tree-optimization/78821 (GCC7: Copying whole 32 bits structure field by field not optimised into copying whole 32 bits at once)

	PR tree-optimization/78821
	* gimple-ssa-store-merging.c (compatible_load_p): Don't require
	that bit_not_p is the same.
	(imm_store_chain_info::coalesce_immediate_stores): Likewise.
	(split_group): Count precisely bit_not_p bits in each statement.
	(invert_op): New function.
	(imm_store_chain_info::output_merged_store): Use invert_op to
	emit BIT_XOR_EXPR with a xor_mask instead of BIT_NOT_EXPR if some
	but not all orig_stores have BIT_NOT_EXPR in the corresponding spots.

	* gcc.dg/store_merging_15.c: New test.

From-SVN: r254679