[dev.typeparams] cmd/compile: dictionary/shape cleanup

 - Removed gcshapeType - we're going with more granular shapes for now, and gradually
   coarsening later if needed.

 - Put in early return in getDictionarySym(), so the entire rest of the
   function can be un-indented by one level.

 - Removed some duplicated infoprint calls, and fixed one infoprint
   message in getGfInfo.

Change-Id: I13acce8fdabdb21e903275b53ff78a1e6a378de2
Reviewed-on: https://go-review.googlesource.com/c/go/+/339901


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