move peepholes where they're relevant

Much easier to reason about in each method.
And extend extract->shr special case to any right shift.

Change-Id: If6af83cf07b2c300cf47816c4c15d717e4d49a8b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218337
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed