a86: Check immediate ranges

[dvanhorn]

Instructions that take immediate values should check that values are in range. Some instructions, e.g. Cmp can only take 32-bit immediate, and will silently do something unexpected if you use a 64-bit immediate.

[dvanhorn]

This is going to be a pain to check. For example, Mov can take a 64-bit immediate, but only if moving into a register, not memory.

[pdarragh]

Regardless of whether we move to the OO technique in #113, I've been planning to revise how instruction AST nodes are defined to make things like this easier to specify, so I'm not sure whether we ought to do it right now in the main branch or not.

[laelath]

I think this is orthogonal to that, since these are just checks on the target language AST

[dvanhorn]

I've added checks in a branch which we can merge later. (Probably after the exam is fine.) I haven't done the "if moving into memory you only get 32-bits" part yet.

[dvanhorn]

Just remembered you can't move immediates into memory, so I think this is done now.