Add `Value.constantAccessPath`. It is like `accessPath`, but ensures that the projectionPath only contains "constant" elements.
This means: if the access contains an `index_addr` projection with a non-constant index, the `projectionPath` does _not_ contain the `index_addr`.
Instead, the `base` is an `AccessBase.index` which refers to the `index_addr`.
* 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
And simplify it.
This struct is not really needed by clients. It's just needed internally in 'Value.accessPath` (and similar properties) to compute the access path.
A projected value consists of the original value and a projection path.
For example, if the `value` is of type `struct S { var x: Int }` and `path` is `s0`, then the projected value represents field `x` of the original value.
Also, use ProjectedValue instead of AccessStoragePath.
It doesn't make sense to let getAccessPathWithScope return an `EnclosingScope` as the second tuple element, because in case it's a `base`, it duplicates the `AccessBase` (which is returned in the first tuple element).
Instead just return an optional `BeginAccessInst` which is not nil if such an "scope" is found.
Now that `AccessBase` is an enum, it makes sense to add an `unidentified` case. This avoids dealing with optional AccessBases in several place.
Clients don't need to make both, an optional check and a switch, but can check for unidentified access bases just in a single switch statement.
