[llvm] [X86] Add MI-layer routine for getting the index of the first address operand. NFC (PR #78019)

[Nicholas Mosier via llvm-commits]

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@@ -289,10 +289,8 @@ static unsigned getYMMtoXMMStoreOpcode(unsigned StoreOpcode) {
 }
 
 static int getAddrOffset(const MachineInstr *MI) {
-  const MCInstrDesc &Descl = MI->getDesc();
-  int AddrOffset = X86II::getMemoryOperandNo(Descl.TSFlags);
+  const int AddrOffset = X86::getFirstAddrOperandIdx(*MI);
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nmosier wrote:

Is it just a stylstic preference? Or why doesn't declaring `AddrOffset` as `const` work here?

https://github.com/llvm/llvm-project/pull/78019