[llvm] AMDGPU: Make v2f32 -> v2f16 legal when target supports v_cvt_pk_f16_f32 (PR #139956)

[Shilei Tian via llvm-commits]

shiltian wrote:

> However, SelectionDAG does not allow us to specify the source type in the legalization rules.

I find it sometimes confusing when I look at those code. The comment of `setOperationAction` says:

```
  /// Indicate that the specified operation does not work with the specified
  /// type and indicate what to do about it. Note that VT may refer to either
  /// the type of a result or that of an operand of Op.
```

Then in what cases it refers to a destination type and in what cases it refers to the source operand type?

https://github.com/llvm/llvm-project/pull/139956