Charge properly for the LDC opcode

[xgreenx]

The LDC opcode benchmark is wrong because $rC represents only the length the users want to load. But if the contract itself is huge, the attacker can use zero $rC and execute very heavy operations for free.

Either the benchmark should work with the worst scenario(the size of the contract is always maximum like ConsensusParameters.contract_max_size) or we need to update the opcode itself and quired exact size of the contract.

[xgreenx]

Fixed by FuelLabs/fuel-vm#598