[llvm] Negative frame indicies as register. (PR #164459)

[Craig Topper via llvm-commits]

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@@ -87,7 +91,7 @@ class Register {
   /// Compute the frame index from a register value representing a stack slot.
   int stackSlotIndex() const {
     assert(isStack() && "Not a stack slot");
-    return static_cast<int>(Reg - Register::FirstStackSlot);
+    return static_cast<int>(SignExtend64(Reg & Register::StackSlotMask, 30));
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topperc wrote:

Use `SignExtend64<30>`

https://github.com/llvm/llvm-project/pull/164459