cmd/compile/internal/types2: check non-generic conversions first

This enables the elimination of convertibleToImpl again,
with the code structure close to the original non-generic
version, and closely matching the structure of assignableTo.

We also don't need the hasTerm tests; instead we can rely
directly on the mechanism of TypeParam.is which is feeding
a nil term if there are no specific types.

Change-Id: I0385acca779d75c3c961d06afb464714fe51705d
Reviewed-on: https://go-review.googlesource.com/c/go/+/361269


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>