[llvm] [ValueTracking] Let ComputeKnownSignBits handle (shl (zext X), C) (PR #97693)

[Björn Pettersson via llvm-commits]

================
@@ -3757,11 +3757,21 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
     }
     case Instruction::Shl: {
       const APInt *ShAmt;
+      Value *X = nullptr;
       if (match(U->getOperand(1), m_APInt(ShAmt))) {
         // shl destroys sign bits.
-        Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
-        if (ShAmt->uge(TyBits) ||   // Bad shift.
-            ShAmt->uge(Tmp)) break; // Shifted all sign bits out.
+        if (ShAmt->uge(TyBits))
+          break; // Bad shift.
+        // We can look through a zext (more or less treating it as a sext) if
+        // all extended bits are shifted out.
+        if (match(U->getOperand(0), m_ZExt(m_Value(X))) &&
+            ShAmt->uge(TyBits - X->getType()->getScalarSizeInBits())) {
+          Tmp = ComputeNumSignBits(X, Depth + 1, Q);
----------------
bjope wrote:

Rebased branch on top of https://github.com/llvm/llvm-project/commit/b8c4c58ecf186dd91f40bdff4d1bdad403435789 to show that the regression no longer can be seen.

https://github.com/llvm/llvm-project/pull/97693