tree-optimization/114074 - CHREC multiplication and undefined overflow

When folding a multiply CHRECs are handled like {a, +, b} * c
is {a*c, +, b*c} but that isn't generally correct when overflow
invokes undefined behavior.  The following uses unsigned arithmetic
unless either a is zero or a and b have the same sign.

I've used simple early outs for INTEGER_CSTs and otherwise use
a range-query since we lack a tree_expr_nonpositive_p and
get_range_pos_neg isn't a good fit.

	PR tree-optimization/114074
	* tree-chrec.h (chrec_convert_rhs): Default at_stmt arg to NULL.
	* tree-chrec.cc (chrec_fold_multiply): Canonicalize inputs.
	Handle poly vs. non-poly multiplication correctly with respect
	to undefined behavior on overflow.

	* gcc.dg/torture/pr114074.c: New testcase.
	* gcc.dg/pr68317.c: Adjust expected location of diagnostic.
	* gcc.dg/vect/vect-early-break_119-pr114068.c: Do not expect
	loop to be vectorized.