[ifcombine] robustify decode_field_reference

Arrange for decode_field_reference to use local variables throughout,
to modify the out parms only when we're about to return non-NULL, and
to drop the unused case of NULL pand_mask, that had a latent failure
to detect signbit masking.


for  gcc/ChangeLog

	* gimple-fold.cc (decode_field_reference): Rebustify to set
	out parms only when returning non-NULL.
	(fold_truth_andor_for_ifcombine): Bail if
	decode_field_reference returns NULL.  Add complementary assert
	on r_const's not being set when l_const isn't.