[llvm] [RISCV] Use inheritance to simplify usage of the UnsupportedSched* multiclasses. NFC (PR #120329)

[Pengcheng Wang via llvm-commits]

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@@ -211,90 +211,57 @@ def ReadFClass16         : SchedRead;
 def ReadFClass32         : SchedRead;
 def ReadFClass64         : SchedRead;
 
+// For CPUs that support Zfhmin, but not Zfh.
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wangpc-pp wrote:

Just a concern, there may be some risks of misuse.

https://github.com/llvm/llvm-project/pull/120329