refactored more SkSL IRNodes

Change-Id: Iab04e4915715bad56b795993022b24a137339daf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324125
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
16 files changed