[Z80] Fix flag liveness tracking during frame index elimination

RegScavenger's isRegUsed() can become unreliable during backward
scavenging in PEI::replaceFrameIndicesBackward because LiveUnits
becomes stale after skipTo() repositions the scavenger without
updating liveness for inserted instructions

This caused incorrect code gen where flag clobbering add instructions
were not wrapped with push and pop whjen flags were actually live

Related: llvm#174251

Author: Sightem

llvm/lib/Target/Z80/Z80InstrInfo.cpp

@@ -1195,7 +1195,33 @@ bool Z80InstrInfo::rewriteFrameIndex(MachineInstr &MI, unsigned FIOperandNum,
   LLVM_DEBUG(dbgs() << "Z80FrameIndex: Offset " << NewOffset << " not legal for "
                     << TRI.getName(BaseReg) << ", RS=" << (RS ? "yes" : "no") << "\n");
 
-  bool SaveFlags = RS && RS->isRegUsed(Z80::F);
+  // determine if we need to preserve flags (Z80::F) around flag clobbering
+  // instructions like ADD. the RegScavenger's isRegUsed() is unreliable during
+  // backward scavenging because its LiveUnits can become stale after skipTo()
+  // in pei::replaceFrameIndicesBackward. we work around this by forward scanning
+  // from the current instruction to find if flags are actually used.
+  // see: https://github.com/llvm/llvm-project/issues/174251
+  auto isFlagLiveForward = [&]() -> bool {
+    if (RS && RS->isRegUsed(Z80::F))
+      return true;
+
+    MachineBasicBlock::iterator ScanStart = II;
+    ++ScanStart;
+    for (MachineBasicBlock::iterator I = ScanStart; I != MBB.end(); ++I) {
+      if (I->definesRegister(Z80::F, &TRI))
+        return false;
+      if (I->readsRegister(Z80::F, &TRI))
+        return true;
+    }
+
+    for (MachineBasicBlock *Succ : MBB.successors())
+      if (Succ->isLiveIn(Z80::F))
+        return true;
+
+    return false;
+  };
+
+  bool SaveFlags = RS && isFlagLiveForward();
   const TargetRegisterClass *OffsetRC =
       Is24Bit ? &Z80::O24RegClass : &Z80::O16RegClass;
   LLVM_DEBUG({