[otbn,sim] BN.MULV instructions cannot be the very first instruction

The simulator models the predecoding of the shuffling index only whilst executing. This is an issue if the very first instruction of a program happens to be one of the BN.MULV instructions. Then the `self.state.mac_rnd_offset_predec` value of `sim.py` hasn't been set properly. We need to set this in the last cycle of `sim._step_pre_exec()` where we leave the state.

Start in sim.py at sim._step_pre_exec(), focusing on the last cycle where the state is left and on self.state.mac_rnd_offset_predec. Reproduce a program whose first instruction is a BN.MULV instruction, then verify that the simulator initializes the shuffling index before executing it.

Written by the indexing model from the issue text.