Add `deletableInstructions()` and `reverseDeletableInstructions()` in SILBasicBlock.
It allows deleting instructions while iterating over all instructions of the block.
This is a replacement for `InstructionDeleter::updatingRange()`.
It's a simpler implementation than the existing `UpdatingListIterator` and `UpdatingInstructionIteratorRegistry`, because it just needs to keep the prev/next pointers for "deleted" instructions instead of the iterator-registration machinery.
It's also safer, because it doesn't require to delete instructions via a specific instance of an InstructionDeleter (which can be missed easily).
This is important for performance diagnostics: it’s assumed that (non-generic) MemoryLayout constants do not need to create metadata at runtime. At Onone this is only guaranteed if the TargetConstantFolding pass runs.
rdar://94836837
* move the source file to SILOptimizer/IRGenTransforms
* add a file level comment
* document and verify that the pass runs after serialization
* catch overflows when truncating a constant value
