[dev.typeparams] cmd/compile: keep instantiated method as a method, rather...

[dev.typeparams] cmd/compile:  keep instantiated method as a method, rather than converting to function

Previously, we were converting an instantitated method to a function, by
moving the receiver arg to the regular args, etc. But that made the type
of the method signature inconsistent with the signature on the method
fields, which leads to some problems with more complex programs with
instantiations. And things work fine if we leave the instantiated method
as a method. So, make the change to keep instantiated methods as real
methods (until they are transformed much later in the compiler).

Change-Id: If34be9e88c1b0ff819d557cf8dfbb31196542e7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/319490


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>