[llvm] [X86][MC] Support decoding of EGPR for APX (PR #72102)

[Phoebe Wang via llvm-commits]

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@@ -636,14 +664,13 @@ static int readModRM(struct InternalInstruction *insn) {
     break;
   }
 
-  reg |= rFromREX(insn->rexPrefix) << 3;
-  rm |= bFromREX(insn->rexPrefix) << 3;
+  reg |= (rFromREX(insn->rexPrefix) << 3) |
+         (r2FromREX2(insn->rex2ExtensionPrefix[1]) << 4);
+  rm |= (bFromREX(insn->rexPrefix) << 3) |
+        (b2FromREX2(insn->rex2ExtensionPrefix[1]) << 4);
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phoebewang wrote:

Do we also need to `| xFromEVEX2of4(insn->vectorExtensionPrefix[1]) << 4`

https://github.com/llvm/llvm-project/pull/72102