middle-end/108625 - wrong folding due to misinterpreted !

The following fixes a problem with ! handling in genmatch which isn't
conservative enough when intermediate simplifications push to the
sequence but the final operation appears to just pick an existing
(but in this case newly defined in the sequence) operand.  The easiest
fix is to disallow adding to the sequence when processing !.

	PR middle-end/108625
	* genmatch.cc (expr::gen_transform): Also disallow resimplification
	from pushing to lseq with force_leaf.
	(dt_simplify::gen_1): Likewise.

	* gcc.dg/pr108625.c: New testcase.