tree-optimization/114672 - WIDEN_MULT_PLUS_EXPR type mismatch

tree-optimization/114672 - WIDEN_MULT_PLUS_EXPR type mismatch

The following makes sure to restrict WIDEN_MULT*_EXPR to a mode
precision final compute type as the mode is used to find the optab
and type checking chokes when seeing bit-precisions later which
would likely also not properly expanded to RTL.

	PR tree-optimization/114672
	* tree-ssa-math-opts.cc (convert_plusminus_to_widen): Only
	allow mode-precision results.

	* gcc.dg/torture/pr114672.c: New testcase.