compiler: type of string index expression is byte

To make this work from the do_type method, add "byte" and "rune" to
the list of known integer types, and look them up that way rather than
via gogo->lookup_global.

For golang/go#8745

Change-Id: Ie63fb3bc0efc1bad240c3d9585ee94eca5fe71ba
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/275653
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>