Revert "moved SkSL ExpressionStatement's data into IRNode"

This reverts commit 6877f0bfdcac49c95ad0856cd0eb77d85c990aa4.

Reason for revert: 
../../src/sksl/SkSLDehydrator.cpp:423:31: error: no member named 'fExpression' in 'SkSL::ExpressionStatement'
                this->write(e.fExpression.get());


Original change's description:
> moved SkSL ExpressionStatement's data into IRNode
>
> Change-Id: I11b1662cd58b01fabba75dbbee40267a62c8b420
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320639
> Reviewed-by: John Stiles <johnstiles@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

TBR=brianosman@google.com,ethannicholas@google.com,johnstiles@google.com

Change-Id: Id22b8a9c93e842b2775e11f5d4c173e25860b5d0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320824
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
10 files changed