[llvm] [RISCV] Add groupid/bitmask for RISC-V extension (PR #94440)

[Kito Cheng via llvm-commits]

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@@ -37,6 +37,13 @@ class RISCVExtension<string name, int major, int minor, string desc,
   bit Experimental = false;
 }
 
+// TODO: Maybe integrate into RISCVExtension class
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kito-cheng wrote:

My first impression is could we integrate into RISCVExtension, and then saw this comment, so let give a try :P

https://github.com/llvm/llvm-project/pull/94440