Ensure that we can index maps whose key types are:
* int
* uint
* int32
* uint32
* int64
* uint64
* uintptr

Fixes #20439

Change-Id: I8fa96b14073c8af72786482ff4ffc3508064ea86
Reviewed-on: https://go-review.googlesource.com/43850
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>