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>
56 files changed