[llvm] [RISCV] Add extension information to RISCVFeatures.td. NFC (PR #89326)

[Pengcheng Wang via llvm-commits]

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@@ -10,115 +10,129 @@
 // RISC-V subtarget features and instruction predicates.
 //===----------------------------------------------------------------------===//
 
+class RISCVExtension<string n, int major, int minor, string f, string v,
+                     string d, list<RISCVExtension> i = []>
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wangpc-pp wrote:

And other single-letter variables? Though they come from Target.td, but I think we should make it verbose so that they are friendly to new comers.

https://github.com/llvm/llvm-project/pull/89326