Uses refactoring

Instead of copying fIndex and marching it forward,
we can tick down our existing uses counts backward,
saving one temporary std::vector.

Our implementation does guarantee the Instructions
returned by users() are sorted, so let's lean into
that... that means we can find the death time of any
instruction simply by looking at users().back()
(if there are any, of course).

Everything else is names and formatting, the biggest
being renaming Uses -> Usage.  There's enough mention
of "users" and "uses" contrasting with each other that
I think it makes sense for the type to have the nice
middle-ground neutral name Usage, reflecting the arrow
and not which way we're thinking about it pointing.

Change-Id: I32ea9af6eb6430a162bee6da4810a599e8ed0dfd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281003
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
3 files changed