PR rtl-optimization/51023
	* combine.c (simplify_comparison) <case SIGN_EXTEND>: Don't use
	val_signbit_known_clear_p for signed comparison narrowing
	optimization.  Don't check for non-VOIDmode, use
	HWI_COMPUTABLE_MODE_P macro.
	<case ZERO_EXTEND>: Don't check for non-VOIDmode.
	Optimize even when const_op is equal to GET_MODE_MASK (mode),
	don't optimize if const_op is negative.

	* gcc.c-torture/execute/pr51023.c: New test.

From-SVN: r181257