small int->Val hunt

I tried making Val a strong enum class typedef
and found that to make code more awkward than it helped.

Here are a few ints I noticed that are really Vals,
i.e. an Instruction index in a program.

Change-Id: I0bcfe26a9ade8053a691ed30af72f8e510a92759
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280636
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
3 files changed