Check if an operand's instruction has been deleted in `UseList.next()`.
This allows to delete an instruction, which has two uses of a value, during use-list iteration.
Type annotations for instruction operands are omitted, e.g.
```
%3 = struct $S(%1, %2)
```
Operand types are redundant anyway and were only used for sanity checking in the SIL parser.
But: operand types _are_ printed if the definition of the operand value was not printed yet.
This happens:
* if the block with the definition appears after the block where the operand's instruction is located
* if a block or instruction is printed in isolation, e.g. in a debugger
The old behavior can be restored with `-Xllvm -sil-print-types`.
This option is added to many existing test files which check for operand types in their check-lines.
Tool selection is primarily done by checking the executable (= symlink) name.
But sometimes (e.g. if the tool symlink is not there) it's useful to have an option for selecting the tool.
The selection option (e.g. -sil-opt) must be the first argument of swift-frontend.
Replace the generic `List` with the (non-generic) `InstructionList` and `BasicBlockList`.
The `InstructionList` is now a bit different than the `BasicBlockList` because it supports that instructions are deleted while iterating over the list.
Also add a test pass which tests instruction modification while iteration.
