jumper, more gathers

jumper, more gathers

This is all the gathers except index 8 and f16, which aren't conceptually hard
but I want to land separately.

Change-Id: I525f2496e55451041bd6ea07985858fda7b56a40
Reviewed-on: https://skia-review.googlesource.com/11524
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>