tree-optimization/114269 - 434.zeusmp regression after SCEV analysis fix

tree-optimization/114269 - 434.zeusmp regression after SCEV analysis fix

The following addresses a performance regression caused by the recent
SCEV analysis fix with regard to folding multiplications and undefined
behavior on overflow.  We do not handle (T) { a, +, b } * c but can
treat sign-conversions from unsigned by performing the multiplication
in the unsigned type.  That's what we already do for additions (but
that misses one case that turns out important).

This fixes the 434.zeusmp regression for me.

	PR tree-optimization/114269
	PR tree-optimization/114074
	* tree-chrec.cc (chrec_fold_plus_1): Handle sign-conversions
	in the third CASE_CONVERT case as well.
	(chrec_fold_multiply): Handle sign-conversions from unsigned
	by performing the operation in the unsigned type.