Yes, help with unit tests would be appreciated.  Especially since it is
not supposed to fix anything, so I'm not sure what unit tests should be
like...

BTW, trying to understand why the patch didn't remove unreachable code
in your first example, I noticed this (both cases are with patch):

def f1():
    return 1
    x()

disassembly:
  3           0 LOAD_CONST               1 (1)
              3 RETURN_VALUE

  4           4 LOAD_GLOBAL              0 (x)
              7 CALL_FUNCTION            0
             10 POP_TOP

def f2():
    return 1
    x()
    return 2

disassembly:
  3           0 LOAD_CONST               1 (1)
              3 RETURN_VALUE

Looking a bit further, I noticed this:
	if (codestr[codelen-1] != RETURN_VALUE)
		goto exitUnchanged;

So, the patch really can't help with your first example, because peephol
optimizer just bails out before real action begins.