Fangrui Song 8b03b3bd4d MIPS: Avoid MCSymbol::isUnset
The unset state will be merged into the undefined state,
so isUnset should be avoided.

The code used by register equating uses the inaccurate condition.
```
.set r1, $4
```

For now just use isUndefined. GNU Assembler has a warning:

```
Warning: register value used as expression
```
2025-08-03 20:03:20 -07:00
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