SkSL enum changes

Changed a couple of SkSL enums to enum classes and rearranged things to
make their storage within IRNode type safe.

Change-Id: I6509d027d79161c1a09473e90943aae061583f20
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324624
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
14 files changed