cmd/compile/internal/types2: split out function instantiation from index expr

Also, factor out recording of type/value information after
evaluating an expression into an operand, so that we can
use it when handling instantiation expressions manually.

Change-Id: I6776e6cc243558079d6a203f2fe0a6ae0ecc33de
Reviewed-on: https://go-review.googlesource.com/c/go/+/308371


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>