Venom fixes

[charles-cooper]

What I did

• fix dft pass (maybe)
• issue variables for literals so they can be reordered via dftpass
• add a heuristic to swap the top of the stack if it's not the next thing to be consumed

How I did it

How to verify it

compile this contract

#pragma experimental-codegen
@external
def foo(x: uint256, y: uint256) -> uint256:
    return x * y

output should include DUP2 DUP2 MUL SWAP1 DUP3 DUP3 DIV EQ SWAP2 ISZERO SWAP1 SWAP2 OR ISZERO which shows that the output of MUL is getting swapped out after it is produced

also, a bunch of things like CALLDATASIZE PUSH1 0x44 SWAP1 LT get reordered so the swaps are removed.

Commit message

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Description for the changelog

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