[InstCombine] Fix funnel shift bailout in demanded bits simplification

We shouldn't simply return here -- we still need to compute the
known bits and fall through to generic handling.

This fixes a -instcombine-verify-known-bits violation in funnel.ll.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -638,8 +638,10 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
           if (auto Opt = convertOrOfShiftsToFunnelShift(*Inst)) {
             auto [IID, FShiftArgs] = *Opt;
             if ((IID == Intrinsic::fshl || IID == Intrinsic::fshr) &&
-                FShiftArgs[0] == FShiftArgs[1])
-              return nullptr;
+                FShiftArgs[0] == FShiftArgs[1]) {
+              computeKnownBits(I, Known, Depth, CxtI);
+              break;
+            }
           }
         }
       }
@@ -718,8 +720,10 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
           if (auto Opt = convertOrOfShiftsToFunnelShift(*Inst)) {
             auto [IID, FShiftArgs] = *Opt;
             if ((IID == Intrinsic::fshl || IID == Intrinsic::fshr) &&
-                FShiftArgs[0] == FShiftArgs[1])
-              return nullptr;
+                FShiftArgs[0] == FShiftArgs[1]) {
+              computeKnownBits(I, Known, Depth, CxtI);
+              break;
+            }
           }
         }
       }