CL: Dedupe inline constant samplers
Fixes #62 (closed).
Clang/llvm generates a unique constant sampler for every read call which references them, rather than based on variable declarations. That causes us to end up with a bunch of identical variables.
This pass dedupes them by pointing all tex ops that reference a constant sampler to the same one, and removing dead derefs. After removing dead uniform variables, we'll only have unique constant samplers.