[dev.typeparams] cmd/compile: simple shape cleanups

 - Changed some early returns to asserts (instantiateMethods and Shapify
   should never take a shape arg)

 - Added suggested change (by Ingo) to use copy() in getInstantiation()

 - Clarified that shape types never have methods in Shapify(), removed
   some TODO comments.

Change-Id: Ia2164ffe670a777f7797bbb45c7ef5e6e9e15357
Reviewed-on: https://go-review.googlesource.com/c/go/+/338971


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>