go/types: factor out index/slice expr handling

go/types: factor out index/slice expr handling

This is a port of CL 308370 to go/types. There are some differences in
the index checking code, but the methodology for moving the code was the
same: replace `goto Error` with `x.mode = invalid; return`.

Change-Id: I880f577a7720e6ad8a5b096207001fcf7620396d
Reviewed-on: https://go-review.googlesource.com/c/go/+/312095
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>