moved SkSL FunctionCall data into IRNode

Change-Id: Iefa82eccbba518cbd741b4415ea6340c66eabd9d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321465
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
17 files changed