cmd/compile/internal/types2: factor out index/slice expr handling

First step towards lightening the load of Checker.exprInternal by
factoring out the code for index and slice expressions; incl. moving
a couple of related methods (Checker.index, Checker.indexedElts).

The code for handling index/slice expressions is copied 1:1 but
occurrences of "goto Error" are replaced by "x.mode = invalid"
followed by a "return".

Change-Id: I44048dcc4851dc5e24f5f169c17f536a37a6a676
Reviewed-on: https://go-review.googlesource.com/c/go/+/308370


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>