compiler: always use int context for index values

For golang/go#14844

Change-Id: Idf1eaf2095d404b01fcfa189453e30195834c266
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273889
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>