[ifcombine] reuse left-hand mask to decode right-hand xor operand

If fold_truth_andor_for_ifcombine applies a mask to an xor, say
because the result of the xor is compared with a power of two [minus
one], we have to apply the same mask when processing both the left-
and right-hand xor paths for the transformation to be sound.  Arrange
for decode_field_reference to propagate the incoming mask along with
the expression to the right-hand operand.

Don't require the right-hand xor operand to be a constant, that was a
cut&pasto.


for  gcc/ChangeLog

	* gimple-fold.cc (decode_field_reference): Add xor_pand_mask.
	Propagate pand_mask to the right-hand xor operand.  Don't
	require the right-hand xor operand to be a constant.
	(fold_truth_andor_for_ifcombine): Pass right-hand mask when
	appropriate.