The inliner's ishairy passes a budget and a reason down through the
walk. Lift these into a visitor object and turn ishairy and its
helpers into methods.

This will make it easy to add more state.

Change-Id: Ic6ae246e1affd67ed283c3205f9595ae33e22215
Reviewed-on: https://go-review.googlesource.com/41151
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>