(simplify_logical, case IOR): Convert back to PLUS if valid and it will
combine with another PLUS.
(extract_left_shift): New function.
(make_compound_operation, case ASHIFTRT): Simplify by calling it.
(force_to_mode): Don't ignore if X is a SUBREG.
(force_to_mode, case AND): Try to turn unchecked bits on instead of just off
and see which is cheaper.

From-SVN: r7224