re PR tree-optimization/15310 ([tree-ssa] Optimize an addition in a bit field)

re PR tree-optimization/15310 ([tree-ssa] Optimize an addition in a bit field)

	* simplify-rtx.c (simplify_binary_operation): Simplify
	((A & N) + B) & M -> (A + B) & M if M is pow2 minus 1 constant and
	N has at least all bits in M set as well.

	PR tree-optimization/15310
	* expr.c (expand_assignment): Optimize += or -= on a bit field in
	most significant bits.

	* gcc.c-torture/execute/20040629-1.c: New test.

From-SVN: r83900