File "/Users/brage/Downloads/vyper/vyper/venom/analysis/liveness.py", line 90, in input_vars_from
    for label, var in inst.phi_operands:
  File "/Users/brage/Downloads/vyper/vyper/venom/basicblock.py", line 374, in phi_operands
    assert isinstance(label, IRLabel), "phi operand must be a label"
AssertionError: phi operand must be a label

%ret_size:3 = phi @8_then, %ret_size:5, @9_else, %ret_size:4

function global {
  global:
    jnz 1, @1_then, @2_then
  1_then:
    %1 = 1
    jmp @3_phi
  2_then:
    %2 = 2
    jmp @3_phi
  3_phi:
    %3 = phi %1, @1_then, %2, @2_then
    return 0,%3
}

function global {
  global:
    jnz 1, @1_then, @2_then
  1_then:
    %1 = 1
    jmp @3_phi
  2_then:
    %2 = 2
    jmp @3_phi
  3_phi:
    %3 = phi %1, @1_then, %2, @2_then
    return 0,%3
}

Fix operands order and clarify opcodes in README for Venom IR

The README incorrectly swapped the arguments for the some of the
opcodes example. This change fixes that and clarifies some of the
opcode usages.