Fix compiler warnings.

[MicahZoltu]

https://github.com/makerdao/dss/blob/17be7db1c663d8069308c6b78fa5c5f9d71134a3/src/vat.sol#L93-L97
https://github.com/makerdao/dss/blob/17be7db1c663d8069308c6b78fa5c5f9d71134a3/src/vat.sol#L108-L110

vat.sol:93:5: Warning: Variable is shadowed in inline assembly by an instruction of the same name
    function add(uint x, int y) internal pure returns (uint z) {
    ^ (Relevant source part starts here and spans across multiple lines).

[MicahZoltu]

Repro case:

pragma solidity 0.5.12;

contract MyContract {
    function add() external pure {
        assembly { }
    }
}