tree-optimization/114121 - chrec_fold_{plus,multiply} and recursion

tree-optimization/114121 - chrec_fold_{plus,multiply} and recursion

The following addresses endless recursion in the
chrec_fold_{plus,multiply} functions when handling sign-conversions.
We only need to apply tricks when we'd fail (there's a chrec in the
converted operand) and we need to make sure to not turn the other
operand into something worse (for the chrec-vs-chrec case).

	PR tree-optimization/114121
	* tree-chrec.cc (chrec_fold_plus_1): Guard recursion with
	converted operand properly.
	(chrec_fold_multiply): Likewise.  Handle missed recursion.

	* gcc.dg/torture/pr114312.c: New testcase.