[llvm] [GlobalISel] Add NumSignBits for G_UNMERGE_VALUES (PR #213607)

[Matt Arsenault via llvm-commits]

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@@ -2511,6 +2511,38 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
       return TyBits - 1; // Every always-zero bit is a sign bit.
     break;
   }
+  case TargetOpcode::G_UNMERGE_VALUES: {
+    unsigned NumOps = MI.getNumOperands();
+    Register SrcReg = MI.getOperand(NumOps - 1).getReg();
+    LLT SrcTy = MRI.getType(SrcReg);
+
+    if ((SrcTy.isVector() && SrcTy.getScalarType() != DstTy.getScalarType()) ||
+        (SrcTy.isScalar() && DstTy.isVector()))
+      break;
+
+    // Figure out the result operand index
+    unsigned DstIdx = 0;
+    for (; DstIdx != NumOps - 1 && MI.getOperand(DstIdx).getReg() != R;
+         ++DstIdx)
+      ;
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arsenm wrote:

This is a pain. I think there's a utility for this somewhere already 

https://github.com/llvm/llvm-project/pull/213607