[llvm] [RISCV][NFC] Allow SchedVar to be a def inside our scheduler model files. (PR #82634)

[Wang Pengcheng via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM.

> For some reason, TableGen does not allow Values to reference records that were created in the same parent record construction.

Yeah, this is a problem. I may make TableGen able to refer to other definitions in a multiclass:
```
multiclass Test {
  def a;
  def b: B<a>; // For `a`, we will search `NAME # a` first and then `a`.
}
```
(Hopefully this can be easy to design and implement)

https://github.com/llvm/llvm-project/pull/82634