cmd/compile/internal/types2: use type variables consistently in Checker.conversion
cmd/compile/internal/types2: use type variables consistently in Checker.conversion

We have V and T and Vu and Tu. When calling the various isX predicates
consistently use Vu and Tu.

(We could also use V an T because the predicates call under anyway,
but using Vu and Tu removes an unnecessary call to Named.under if
V or T are *Named.)

Also, removed some outdated comments.

Change-Id: I6fcd9ce5f6292e89ac2afd597b72fd0790e84ff1
Reviewed-on: https://go-review.googlesource.com/c/go/+/362895
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go