[ifcombine] drop other misuses of uniform_integer_cst_p

As Jakub pointed out in PR118206, the use of uniform_integer_cst_p in
ifcombine makes no sense, we're not dealing with vectors.  Indeed,
I've been misunderstanding and misusing it since I cut&pasted it from
some preexisting match predicate in earlier version of the ifcombine
field-merge patch.


for  gcc/ChangeLog

	* gimple-fold.cc (decode_field_reference): Drop misuses of
	uniform_integer_cst_p.
	(fold_truth_andor_for_ifcombine): Likewise.