cmd/vet: type conversions never have side effects

cmd/vet: type conversions never have side effects

Make the hasSideEffects func use type information to see if a CallExpr
is a type conversion or not. In case it is, there cannot be any side
effects.

Now that vet always has type information, we can afford to use it here.
Update the tests and remove the TODO there too.

Change-Id: I74fdacf830aedf2371e67ba833802c414178caf1
Reviewed-on: https://go-review.googlesource.com/79536
Reviewed-by: Robert Griesemer <gri@golang.org>