refactor extra sugared values (#26270)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26270
We've accumulated a lot of sugared values whose only purpose is
to be instanced-checked against in emitApplyExpr. I need to add
another one to insert an unchecked_cast, and do not want to continue
the pattern. This creates an abstraction for this concept (SpecialFormValue),
and removes all the unneeded sugared values. There is no functionality
change here just a bunch of code movement in compiler.cpp
Test Plan: Imported from OSS
Differential Revision: D17412854
Pulled By: zdevito
fbshipit-source-id: 15877c91decaea5a00d1fe737ed2d0f0f8a79a28
