Simplify X * C1 == C2 with undefined overflow

Simplify X * C1 == C2 with undefined overflow

this transformation is quite straightforward, without overflow, 3*X==15 is
the same as X==5 and 3*X==5 cannot happen. Adding a single_use restriction
for the first case didn't seem necessary, although of course it can
slightly increase register pressure in some cases.

2020-08-04  Marc Glisse  <marc.glisse@inria.fr>

	PR tree-optimization/95433
	* match.pd (X * C1 == C2): New transformation.

	* gcc.c-torture/execute/pr23135.c: Add -fwrapv to avoid
	undefined behavior.
	* gcc.dg/tree-ssa/pr95433.c: New file.