moved SkSL FieldAccess data into IRNode

Change-Id: Ib2fd8c246799a1bde566395080fe6617754644f9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324635
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
16 files changed