Fix compiler warnings.

[MicahZoltu]

dss/src/vat.sol

Lines 93 to 97 in 17be7db

	function add(uint x, int y) internal pure returns (uint z) {
	z = x + uint(y);
	require(y >= 0 || z <= x);
	require(y <= 0 || z >= x);
	}

dss/src/vat.sol

Lines 108 to 110 in 17be7db

	function add(uint x, uint y) internal pure returns (uint z) {
	require((z = x + y) >= x);
	}

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 { }
    }
}