tree-optimization/94234 - add plusminus-with-convert pattern

tree-optimization/94234 - add plusminus-with-convert pattern

Add a rule (T)(A) +- (T)(B) -> (T)(A +- B), which works only when (A +- B)
could be folded to a simple value. By this rule, a plusminus-mult-with-convert
expression could be handed over to the rule (A * C) +- (B * C) -> (A +- B).

2020-09-15  Feng Xue  <fxue@os.amperecomputing.com>

gcc/
	PR tree-optimization/94234
	* match.pd (T)(A) +- (T)(B) -> (T)(A +- B): New simplification.

gcc/testsuite/
	PR tree-optimization/94234
	* gcc.dg/pr94234-3.c: New test.