[dev.typeparams] cmd/compile: remove remaining uses of Unshapify

The other uses of Unshapify were really only there to allow for the
dictionary checking code at the beginning of generic functions/methods.
But that will go away as soon as we start combining real shapes. If we
get rid of that code, we can get rid of the unshapify calls elsewhere.

The only tricky part is that getInstantiation now gets targs that may each
either be a shape or concrete type, and it must translate any concrete
types to shapes, while leaving the already existing shapes.

Change-Id: Ib2b9072b921f8e064958548a1078d82f1d040c9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/338289


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>