cmd/compile: remove now-unneeded SetHasTParam() for cached ptr element

We had code in NewPtr() to set the HasTParam/HasShape flag as needed for
the cached ptr element if it wasn't set correctly based on its Elem.
This was causing the race mentioned in the issue.

But that setting code is no longer needed, as long as we call
SetRParams() soon after calling NewIncompleteNamedType(), before
creating/translating the underlying type (which we do). The
HasTParam/HasShape attribute can only come from setting of rparams or a
direct typeparam/shape somewhere in the underlying type, both of which
don't depend on recursion, etc. (as long as the rparams are set early).
Added a check that HasTParam/HasShape are set correctly for the cached
pointer/slice elems in NewPtr() and NewSlice().

Fixes #48191

Change-Id: Ide7d82efb77ae97901e75b2e6c65bd1bfc25e0ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/348089


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>