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
```

Author: MaskRay

llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp

@@ -6654,7 +6654,7 @@ bool MipsAsmParser::searchSymbolAlias(OperandVector &Operands) {
           llvm_unreachable("Should never fail");
       }
     }
-  } else if (Sym->isUnset()) {
+  } else if (Sym->isUndefined()) {
     // If symbol is unset, it might be created in the `parseSetAssignment`
     // routine as an alias for a numeric register name.
     // Lookup in the aliases list.