Fix parsing error with SPIR-V negating a uint.

Our SPIR-V code generator did not implement support for negating a uint.
However, this is something that GLSL allows (as does the rest of SkSL).
I checked glslang and it uses OpSNegate here. The SPIR-V docs indicate
that OpSNegate allows any type of integer, and the validator lets it
pass, so we now use OpSNegate here as well.

http://screen/33mkq92uxAT5Xu8
http://screen/4YBTh3gCWz8eZx7
http://screen/388HtXyytcN5vLZ

Change-Id: I8c142018fd5e162dcd051abe1bc5d69a6e034794
Bug: oss-fuzz:37627
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441880
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
4 files changed