[DAG] SimplifyDemandedBits - ignore SRL node if we're just demanding known sign bits (#114805)

Check to see if we are only demanding (shifted) signbits from a SRL node that are also signbits in the source node.

We can't demand any upper zero bits that the SRL will shift in (up to max shift amount), and the lower demanded bits bound must already be all signbits.

Same fold as #114389 which added this for SimplifyMultipleUseDemandedBits

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -2030,6 +2030,21 @@ bool TargetLowering::SimplifyDemandedBits(
       Known = TLO.DAG.computeKnownBits(Op, DemandedElts, Depth);
     }
 
+    // If we are only demanding sign bits then we can use the shift source
+    // directly.
+    if (std::optional<uint64_t> MaxSA =
+            TLO.DAG.getValidMaximumShiftAmount(Op, DemandedElts, Depth + 1)) {
+      unsigned ShAmt = *MaxSA;
+      // Must already be signbits in DemandedBits bounds, and can't demand any
+      // shifted in zeroes.
+      if (DemandedBits.countl_zero() >= ShAmt) {
+        unsigned NumSignBits =
+            TLO.DAG.ComputeNumSignBits(Op0, DemandedElts, Depth + 1);
+        if (DemandedBits.countr_zero() >= (BitWidth - NumSignBits))
+          return TLO.CombineTo(Op, Op0);
+      }
+    }
+
     // Try to match AVG patterns (after shift simplification).
     if (SDValue AVG = combineShiftToAVG(Op, TLO, *this, DemandedBits,
                                         DemandedElts, Depth + 1))