Improve push ir conversion

[samczsun]

First attempt at handling arr.push() better. Might have used ReferenceVariable wrong. Also removed a block about pushing an array because I'm not sure what Solidity code might trigger that. Hopefully fixes #593

                Expression: arr.push().push()
                IRs:
                        REF_2 -> LENGTH arr
                        TMP_2(uint256) := REF_2(uint256)
                        TMP_3(uint256) = TMP_2 + 1
                        REF_2 (->arr) := TMP_3(uint256)
                        REF_3(uint256[]) -> arr[TMP_2]
                        TMP_0(uint256[]) := REF_3(uint256[])
                        REF_4 -> LENGTH TMP_0
                        TMP_4(uint256) := REF_4(uint256)
                        TMP_5(uint256) = TMP_4 + 1
                        REF_4 (->TMP_0) := TMP_5(uint256)
                        REF_5(uint256) -> TMP_0[TMP_4]
                        TMP_1(uint256) := REF_5(uint256)

[montyly]

So the array path is needed for something like:

contract C{

    uint[][] arr;

    function f() public{
        arr.push([1,2]);
    }

}

In master it generates:

			TMP_1(None) =  ['1', '2']
			PUSH TMP_1 in arr
			TMP_0 -> LENGTH arr

In bugfix/593, it triggers an error: ERROR:ContractSolcParsing:Impossible to generate IR for C.f

[samczsun]

                Expression: arr.push((1,2))
                IRs:
                        REF_7 -> LENGTH arr
                        TMP_7(uint256) := REF_7(uint256)
                        TMP_8(uint256) = TMP_7 + 1
                        REF_7 (->arr) := TMP_8(uint256)
                        REF_8(uint256[]) -> arr[TMP_7]
                        TMP_9(uint256[]) =  ['1', '2']
                        REF_8 (->arr) := TMP_9(uint256[])