Lower PanicLN, CJumpLN and JumpLN.
Rename getVars(Read|Written) to getHVars(Read|Written).

We need to introduce a getLVars(Read|Written) without conflict.
Add a "null" register allocator.

The null register allocator simply spills all virtual registers to the
stack and inserts appropriate stores and loads.  This will stay in place
till the first compiled program runs.