[PATCH] Fixing inst-combine not to drops nsw when combining adds into mul (PR19263)

[Rafael Espíndola]

> Here both constants are from different expression, one from LHS and other
> one is from RHS.

All tests still pass if I delete the extra logic from
WillNotOverflowSignedAdd. Can you add an extra test of split this off
to an independent patch?

Cheers,
Rafael