Simplify X * C1 == C2 with wrapping overflow

Simplify X * C1 == C2 with wrapping overflow

Odd numbers are invertible in Z / 2^n Z, so X * C1 == C2 can be rewritten
as X == C2 * inv(C1) when overflow wraps.

mod_inv should probably be updated to better match the other wide_int
functions, but that's a separate issue.

2020-08-10  Marc Glisse  <marc.glisse@inria.fr>

	PR tree-optimization/95433
	* match.pd (X * C1 == C2): Handle wrapping overflow.
	* expr.c (maybe_optimize_mod_cmp): Qualify call to mod_inv.
	(mod_inv): Move...
	* wide-int.cc (mod_inv): ... here.
	* wide-int.h (mod_inv): Declare it.

	* gcc.dg/tree-ssa/pr95433-2.c: New file.