Fix debugger cursor jumping on some Morley instructions

Description

Steps to reproduce

1. Open verbose-typecheck/letblock.mtz in the debugger.
2. Set the parameter to Unit.
3. Set the storage to Unit.
4. Start the debugger.
5. Press "Step Over" four times. The debugger will execute add next.
6. Press "Step Over" one time. The debugger jumps to the definition of add without stopping at the beginning of the add instruction in the code block.
7. Press "Step over" one time. The cursor should return to the code block at the beginning of the stacktype instruction, but instead goes to the definition of the stkRest.

Expected behaviour

1. The cursor doesn't ignore stacktype.
2. Before executing the definition of add, and the definition of stkRest, the cursor should stop at the beginning of each of these instructions.

Actual behaviour

1. The cursor ignores stacktype.
2. The cursor jumps directly to the definitions of add and stkRest.