AD Cleanup: Cleanup slang-ir-autodiff-unzip.cpp & slang-ir-autodiff-unzip.h

[saipraveenb25]

slang-ir-autodiff-unzip.cpp currently contains very little of the actual unzipping logic and instead contains the extraction pass logic.
This is tech-debt from earlier when we decided to restructure the passes so that extraction/scope-legalization occured at the end rather than after unzipping.

We should move passes into appropriately named files and move the implementations from the header to the .cpp module.