* split the `PassContext` into multiple protocols and structs: `Context`, `MutatingContext`, `FunctionPassContext` and `SimplifyContext`
* change how instruction passes work: implement the `simplify` function in conformance to `SILCombineSimplifyable`
* add a mechanism to add a callback for inserted instructions
To add a module pass in `Passes.def` use the new `SWIFT_MODULE_PASS` macro.
On the swift side, create a `ModulePass`.
It’s run function receives a `ModulePassContext`, which provides access to all functions of a module.
But it doesn't provide any APIs to modify functions.
In order to modify a function, a module pass must use `ModulePassContext.transform(function:)`.
