simplify-rtx.c (simplify_const_binary_operation): Use the shift_truncation_mask hook instead of performing modulo by width.

	* simplify-rtx.c (simplify_const_binary_operation): Use the
	shift_truncation_mask hook instead of performing modulo by
	width.  Compare against mode precision, not bitsize.
	* combine.c (combine_simplify_rtx, simplify_shift_const_1):
	Use shift_truncation_mask instead of constructing the value
	manually.

From-SVN: r175913