cmd/compile/internal/types2: simplify Checker.funcInst

Now that we use square brackets for instantiations, we
can tell type arguments from ordinary arguments without
"guessing" which permits a simpler implementation.

While at it, also fix a minor position error for type
instantiations (now matching the code for function
instantiations).

Change-Id: I20eca51c5b06259703767b5906e89197d6cd595a
Reviewed-on: https://go-review.googlesource.com/c/go/+/306169


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>