tree-optimization/94234 - Fold plusminus_mult expr with multi-use operands

tree-optimization/94234 - Fold plusminus_mult expr with multi-use operands

2020-09-03  Feng Xue  <fxue@os.amperecomputing.com>

gcc/
	PR tree-optimization/94234
	* genmatch.c (dt_simplify::gen_1): Emit check on final simplification
	result when "!" is specified on toplevel output expr.
	* match.pd ((A * C) +- (B * C) -> (A +- B) * C): Allow folding on expr
	with multi-use operands if final result is a simple gimple value.

gcc/testsuite/
	PR tree-optimization/94234
	* gcc.dg/pr94234-2.c: New test.