Opt: Optimize Private variable references

Currently, spirv-opt only optimizes references for function scope variables. However, there exist shaders where optimizing Private (static) variables can have benefits for following optimizations such as dead branch elimination, etc.

Start by tracing the Private variable reference optimization path in spirv-opt and compare it with the existing function-scope variable handling. Investigate how the result affects later optimizations such as dead branch elimination. Done means Private (static) variable references are optimized without changing shader behavior, with coverage for the reported shader cases.

Written by the indexing model from the issue text.