fix[docs]: fix venom examples

[2xic]

What I did

I was playing around with the venom IR parser and ran into issues when using some of the instructions per the README. The current docs has swapped the usage of labels and the vars a few places.

How I did it

I got a error message which hinted at the issue

  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

Then I ran the following on a Vyper file which would use the phi function to find a reference

 vyper --experimental-codegen -f bb_runtime 

It gave out

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

How to verify it

Per the docs, this should compile, but it doesn't

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
}

With my suggested fix it does compile

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
}

Using python3 -m vyper.cli.venom_main [file] to verify. Same is true for jnz and the example code.

Commit message

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Description for the changelog

Fix operands order for venom instruction in README example  

The README incorrectly swapped the arguments for the phi instruction  
example. This fix updates the documentation to match the correct  
instruction usage.  

Cute Animal Picture

[charles-cooper]

@2xic is this ready for review?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.12%. Comparing base (c75a2da) to head (33e9e3c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4475   +/-   ##
=======================================
  Coverage   92.12%   92.12%           
=======================================
  Files         119      119           
  Lines       16967    16967           
  Branches     2872     2872           
=======================================
  Hits        15631    15631           
  Misses        918      918           
  Partials      418      418           

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