more SkSL IRNode refactoring

Prefix, Postfix, and ExternalFunctionCall nodes this time around.

Change-Id: I56bc06d73274f01b67f043a6ebd23dd4c80d16e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324621
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
15 files changed