[llvm] [X86][NFC] Reorder the registers to reduce unnecessary iterations (PR #70222)

[Phoebe Wang via llvm-commits]

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@@ -1182,11 +1182,27 @@ namespace X86II {
     }
   }
 
+  inline bool isXMMReg(unsigned RegNo) {
+    return (RegNo >= X86::XMM0 && RegNo <= X86::XMM15) ||
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phoebewang wrote:

Add comment to explain the reason, besides, add assert to make sure they are continuous, e.g.,

`assert(X86::XMM15 - X86::XMM0 == 15, "XMM registers are not continuous");`

https://github.com/llvm/llvm-project/pull/70222