sim: pru: Fix behaviour when loop count is zero

sim: pru: Fix behaviour when loop count is zero

If the counter for LOOP instruction is provided by a register with
value zero, then the instruction must cause a PC jump directly to the
loop end.  But in that particular case simulator must not initialize
its internal loop variables, because loop body will not be executed.
Instead, simulator must obtain the loop's end address directly from
the LOOP instruction.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>