Function return values misplaced by loop hoisting

[cardillan]

The loop hoisting optimization mistakenly processes assignments from function return variables. This code

#set function-inlining = off

while true
    a = foo()
    b = foo()
    print(a, b)
end

def foo()
    rand(10)
end

produces

set a __fn0retval
set __fn0retaddr 3
jump 8 always 0 0
set __fn0retaddr 5
jump 8 always 0 0
print a
print __fn0retval
jump 1 always 0 0
op rand __fn0retval 10 0
set @counter __fn0retaddr

The set a __fn0retval instruction was hoisted in front of the loop - the optimizer doesn't recognize the __fn0retval value is actually produced by the function being called.