cmd/compile: clean up slice and string offsets/sizes

cmd/compile: clean up slice and string offsets/sizes

Minor cleanup:

* Modernize comments.
* Change from int to int64 to avoid conversions.
* Use idiomatic names.

Passes toolstash-check.

Change-Id: I93560c81926c0f4e00f33129cb4846b53bea99e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/227548
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>