Moved SkSL VariableReference data into IRNode

As part of this change, broke up IRNode::type() and moved it into
virtual functions where it belongs.

Change-Id: Ib19e99f2e8392e34a20a1887b980225ac4b793f7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/323884
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
27 files changed