Currently, complex operands of an instruction are flattened in the resulting DAG of `InstAlias`.
This change makes it required to specify complex operands in `InstAlias` as sub-DAGs:
```
InstAlias<"foo $rd, $rs1, $rs2", (Inst RC:$rd, (ComplexOp RC:$rs1, GR0, 42), SimpleOp:$rs2)>;
```
instead of
```
InstAlias<"foo $rd, $rs1, $rs2", (Inst RC:$rd, RC:$rs1, GR0, 42, SimpleOp:$rs2)>;
```
The advantages of the new syntax are improved readability and more robust type checking, although it is a bit more verbose.
981f25a8a8