Reland "Moved SkSL data back into node classes"

This is a reland of f71e0be970978f04b4670b41c12ad0baffbb7fcd

Original change's description:
> Moved SkSL data back into node classes
>
> The original goal of this rearchitecture had been to move all of the
> data into IRNode so that we could manage IRNode objects directly rather
> than std::unique_ptr<IRNode>. Other changes have rendered that original
> goal obsolete, so this is undoing most of the work that was done during
> this rearchitecture.
>
> Change-Id: Ic56ffb17bb013c8b4884d710215f5345a481468a
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330297
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>
> Reviewed-by: John Stiles <johnstiles@google.com>

Change-Id: Ifec4777a42ef0f95f6edc418dcd46fd38c856fa5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330739
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
56 files changed