Optimize `is_changed_arg` unification table ops

In `is_changed_arg` we do four variations on this (pseudocode-ish)
expression:
```
!(probe_value(vid) == Unknown && find(vid) == vid)
```
The first sub-expression means "the var's value is unknown" and the
second means "this var is a root". `probe_value` and `find` are both
parent-chasing operations, i.e. they duplicate some of their work.

We can instead use `try_probe_value` to do this more cheaply:
```
try_probe_value(vid) != Some(Unknown)
```
This has the identical meaning because `try_probe_value` means "get the
var's value, but only if it's a root". (Strange but fortuitous!)

There are four cases like this in `is_changed_arg`. Three of them
(int/float/const vars) are very similar to each other. The ty var case
has slightly different syntax but it's the same idea.