[llvm] [SelectionDAG] Move HwMode expansion from tablegen to SelectionISel. (PR #174471)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@topperc I'm seeing a lot of these warnings:
```
E:\llvm\ninja\lib\Target\PowerPC\PPCGenDAGISel.inc(135470): error C2220: the following warning is treated as an error
E:\llvm\ninja\lib\Target\PowerPC\PPCGenDAGISel.inc(135470): warning C4065: switch statement contains 'default' but no 'case' labels
```
which seem to be from default-only switch statements inside getValueTypeForHwMode:
```
  case 2:
    switch (HwMode) {
    default:
      return MVT::i32;
    }
    break;
```

https://github.com/llvm/llvm-project/pull/174471