tree-ssa-math-opts.c (convert_plusminus_to_widen): Restrict handling of signedness differences to integer types.

gcc/
	* tree-ssa-math-opts.c (convert_plusminus_to_widen): Restrict
	handling of signedness differences to integer types.  Only build
	a new optype if type1 isn't correct.

From-SVN: r182554