Fix strange issue with reassigned argument values

[dstelzer]

Fixes #204

See there for explanation

[bkirwi]

The linked issue does a great job of describing the issue, but the fix seems to me just as obscure; any brief description of why this fixes that?

[dstelzer]

It's a weird one! Basically, when compiling IR, Dialog tries to use as few temporary registers as possible. As part of this, it keeps an array called known_args that holds the known values of all the argument registers. If it needs to load a constant into a register, and one of the known_args already holds that constant, it uses that argument register instead of a new temporary.

Except in a certain edge case, the value of the argument register could change, but the slot in known_args would not be updated. For example:

(test 1)
    (show [[1] [2]])

When (test $) is called, its argument 1 is put into argument register A0, and it needs to put the list [[1] [2]] into A0 for the (show $) call. As it's building that list, known_args for A0 still has the value 1, so it tries to use that to construct the list. But A0 has already been overwritten by [2], so it ends up constructing [[[2]] [2]] instead.

This PR fixes the problem by updating the known_args in that edge case: putting in a new value if it's a compile-time constant, or just setting it to NULL if it's not.