[llvm] [RISC-V] Fix incorrect epilogue_begin (PR #120623)

[Sam Elliott via llvm-commits]

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@@ -1088,7 +1092,8 @@ void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
   // Skip to before the restores of scalar callee-saved registers
   // FIXME: assumes exactly one instruction is used to restore each
   // callee-saved register.
-  auto LastFrameDestroy = std::prev(MBBI, getUnmanagedCSI(MF, CSI).size());
+  auto FirstScalarCSRRestoreInsn =
+      std::next(MBBI, getRVVCalleeSavedInfo(MF, CSI).size());
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lenary wrote:

I don't fully understand why this iterator is now different, and how this behaviour is better. Can you explain a bit more how you know this is correct?

https://github.com/llvm/llvm-project/pull/120623