AMDGPU: Don't use MUBUF vaddr if address may overflow

Effectively revert r263964. Before we would not
allow this if vaddr was not known to be positive.

llvm-svn: 318240
diff --git a/llvm/lib/Target/AMDGPU/SIISelLowering.h b/llvm/lib/Target/AMDGPU/SIISelLowering.h
index 7fab274..f68f7dc 100644
--- a/llvm/lib/Target/AMDGPU/SIISelLowering.h
+++ b/llvm/lib/Target/AMDGPU/SIISelLowering.h
@@ -277,6 +277,12 @@
                    SDValue V) const;
 
   void finalizeLowering(MachineFunction &MF) const override;
+
+  void computeKnownBitsForFrameIndex(const SDValue Op,
+                                     KnownBits &Known,
+                                     const APInt &DemandedElts,
+                                     const SelectionDAG &DAG,
+                                     unsigned Depth = 0) const override;
 };
 
 } // End namespace llvm