Use unique variable name for yul variable

[montyly]

Add a unique name for yul variables to prevent lookup issue (fix #647)

Testcase:

contract C{
    function f() public{
        if(true) {
            assembly{
                let a := 10
                for {} true {} {
                    a := add(a, 1)
                }
            }
        }
        else{
            assembly{
                let a := 10
                for {} true {} {
                    a := add(a, 1)
                }
            }
        }
    }
}