[llvm] [CodeGen] Add Register::stackSlotIndex(). Replace uses of Register::stackSlot2Index. NFC (PR #125028)

[Matt Arsenault via llvm-commits]

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@@ -98,6 +98,12 @@ class Register {
   /// register in a function will get the index 0.
   unsigned virtRegIndex() const { return virtReg2Index(Reg); }
 
+  /// Compute the frame index from a register value representing a stack slot.
+  int stackSlotIndex() const {
+    assert(isStack() && "Not a stack slot");
+    return int(Reg - MCRegister::FirstStackSlot);
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arsenm wrote:

```suggestion
    return static_cast<int>(Reg - MCRegister::FirstStackSlot);
```

https://github.com/llvm/llvm-project/pull/125028