fixed SkSL interpreter negate op

Bug: skia:
Change-Id: I76ff729a887948119ec1ef58221433a6d91e98a8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212484
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
3 files changed