[llvm] AArch64: Mark status flag clobbers dead in SME ABI pass (PR #228042)

[Matt Arsenault via llvm-commits]

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@@ -767,7 +767,8 @@ PhysRegSave MachineSMEABI::createPhysRegSave(LiveRegs PhysLiveRegs,
     RegSave.StatusFlags = MRI->createVirtualRegister(&AArch64::GPR64RegClass);
     BuildMI(MBB, MBBI, DL, TII->get(AArch64::MRS), RegSave.StatusFlags)
         .addImm(AArch64SysReg::NZCV)
-        .addReg(AArch64::NZCV, RegState::Implicit);
+        .addReg(AArch64::NZCV, RegState::Implicit)
+        .setOperandDead(2); // implicit-def $nzcv
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arsenm wrote:

The comment says this:
```
  // The MRS is set as a NZCV setting instruction. Not all MRS instructions
  // require doing this. The alternative was to explicitly model each one, but
  // it feels like it is unnecessary because it seems there are no negative
  // consequences setting these flags for all.
```


https://github.com/llvm/llvm-project/pull/228042