[llvm] [Reland][X86] Remove redundant `cmp` before `adc` after `lzcnt` or `tzcnt` (PR #210069)

[Phoebe Wang via llvm-commits]

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@@ -5619,6 +5647,19 @@ bool X86InstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
       ShouldUpdateCC = true;
     }
 
+    if (LTZCNTInst) {
+      unsigned InstCode = Instr.getOpcode();
+      bool UsesOnlyCF = X86::isADC(InstCode) || X86::isSBB(InstCode) ||
+                        X86::isRCL(InstCode) || X86::isRCR(InstCode) ||
+                        OldCC == X86::COND_B || OldCC == X86::COND_AE;
+      if (!UsesOnlyCF)
+        return false;
+
+      MI = LTZCNTInst;
+      FlagsMayLiveOut = false;
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phoebewang wrote:

Why set it false?

https://github.com/llvm/llvm-project/pull/210069