Dead-end blocks are blocks from which there is no path to the function exit (`return`, `throw` or unwind).
These are blocks which end with an unreachable instruction and blocks from which all paths end in "unreachable" blocks.
Provides a list of instructions, which reference a function.
A function "use" is an instruction in another (or the same) function which references the function.
In most cases those are `function_ref` instructions, but can also be e.g. `keypath` instructions.
'FunctionUses' performs an analysis of all functions in the module and collects instructions which reference other functions.
This utility can be used to do inter-procedural caller-analysis.
These sets are _much_ more efficient than `Set<Value>` and `Set<Instruction>` because they bridge to the efficient `NodeSet`.
Insertions/deletions are just bit operations.
* add `BasicBlockSet`
* add `BasicBlockWorklist`
* add `BasicBlockRange`, which defines a range of blocks from a common dominating “begin” block to a set of “end” blocks.
* add `InstructionRange`, which is similar to `BasicBlockRange`, just on instruction level. It can be used for value lifetime analysis.
* rename `StackList` -> `Stack` and move it to `Optimizer/DataStructures`
* rename `PassContext.passContext` to `PassContext._bridged`
* add notify-functions to PassContext
