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.
Refactor of the llvm-tblgen source into:
- a "Basic" library, which contains the bare minimum utilities to build
`llvm-min-tablegen`
- a "Common" library which contains all of the helpers for TableGen
backends. Such helpers can be shared by more than one backend, and even
unit tested (e.g. CodeExpander is, maybe we can add more over time)
Fixes#80647
