[dev.typeparams] cmd/compile: get runtime stuff working

Remaining stuff from Dan's CL 335412 to get tests passing.

 - Removed PTRLIT case in node() that was added with the gcshape change.
   I don't think it is needed anymore.

 - Modified (*itab).init() to add an irrelevant pointer (the itab
   itself) for the '==' entry of interfaces with 'comparable'. That
   entry is not used, so we just want to avoid the error that the given
   type doesn't actually have an '==' method.

Change-Id: I9b2d4c8342c0b2048d76e003d95023f4ccd559f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/336149


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