go/types: cleanup and fix Checker.index

A couple minor spec compliance issues: constant, typed index operands
must still be representable as type "int", but should also be recorded
as their original type.

Fixes #45667.

Change-Id: Iefeb29f20a8e48350af83a62c9ae0e92198c5ef7
Reviewed-on: https://go-review.googlesource.com/c/go/+/312591


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