Fix for integer wraparound in sksl

I found the ES 2 spec [1] helpful for reference here.

The calculate_count_neq_int is not strictly necessary (I was unable
to find a case that tricked the existing floats with ints), but
I like the refactoring and it mirrors the gt/lt cases nicely.

[1] https://registry.khronos.org/OpenGL/specs/es/2.0/GLSL_ES_Specification_1.00.pdf

Change-Id: I0b9117f347e4b7d5d336de0f14337b9bec510ff2
Bug: 513337118
Fixed: 513337118
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1236656
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>