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

[Phoebe Wang via llvm-commits]

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@@ -149,7 +170,12 @@ class X86OpcodePrefixHelper {
     setR(getRegEncoding(MI, OpNum));
   }
   void setX(const MCInst &MI, unsigned OpNum, unsigned Shift = 3) {
-    X = getRegEncoding(MI, OpNum) >> Shift & 1;
+    unsigned Reg = MI.getOperand(OpNum).getReg();
+    // X is used to extend vector register only when shift is not 3
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phoebewang wrote:

Missing the ending `.` in this and below comments.

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