[llvm] [X86][MC] Support encoding of EGPR for APX (PR #71909)

[Phoebe Wang via llvm-commits]

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@@ -139,7 +147,20 @@ class X86OpcodePrefixHelper {
   }
 
   void setR(unsigned Encoding) { R = Encoding >> 3 & 1; }
-  void setR2(unsigned Encoding) { EVEX_R2 = Encoding >> 4 & 1; }
+  void setR2(unsigned Encoding) {
+    R2 = Encoding >> 4 & 1;
+    assert((!R2 || (Kind <= REX2 || Kind == EVEX)) && "invalid setting");
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phoebewang wrote:

`Kind <= REX2` includes `Kind == None` which doesn't seem correct.
The same for below.

https://github.com/llvm/llvm-project/pull/71909