Incorrect loop hoisting

[cardillan]

Loop hoisting may hoist assignments in front of the loop even when the loop is not guaranteed to run:

#set target = 8;

param a = 10;
b = a;
while rand(10) > 10 do
    b = 2 * a;
    print(b);
end;
print(b);

compiles to

    set a 10
    op mul :b 2 a
    op rand *tmp0 10 0
    jump label_7 lessThanEq *tmp0 10
label_4:
        print :b
    op rand *tmp0 10 0
    jump label_4 greaterThan *tmp0 10
    label_7:
    print :b

This loop doesn't execute (obviously), but the program prints 20.

[cardillan]

There will be a partial fix for now: if the loop isn't known to be executed at least once, only instructions setting up variables not used outside of the loop will be hoisted.

It's possibly ineffective, because the hoisted instructions might be executed even though the loop itself isn't, but at least it's not buggy. A full solution will require updating the loop rotation optimization, and possibly merging the loop hoisting/loop rotation optimizations together (as partial loop rotation won't be available with loop hoisting, so these two optimizations will probably have to be considered together).