[llvm] [PowerPC] Utilize `getReservedRegs` to find asm clobberable registers. (PR #99766)

[Chen Zheng via llvm-commits]

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@@ -2762,7 +2762,7 @@ void PPCFrameLowering::updateCalleeSaves(const MachineFunction &MF,
     // Get the lowest numbered register for each class that actually needs
     // to be saved.
     MCPhysReg Cand = CSRegs[i];
-    if (!SavedRegs.test(Cand))
+    if (!SavedRegs.test(Cand) || Cand == PPC::X2)
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chenzheng1030 wrote:

nit: a comment here to indicate why X2 needs to be excluded? And also why R2 does not need to be execluded?

https://github.com/llvm/llvm-project/pull/99766