This converts all remaining uses of mspan.start to instead use
mspan.base(). In many cases, this actually reduces the complexity of
the code.

Change-Id: If113840e00d3345a6cf979637f6a152e6344aee7
Reviewed-on: https://go-review.googlesource.com/22590Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>