DAG: Add computeKnownBitsForFrameIndex

[arsenm]

Summary

Some of the AMDGPU stack addressing modes require knowing the sign
bit is zero. We used to accomplish this by custom lowering
frame indexes, and then putting an AssertZext around a
TargetFrameIndex. This required specifically looking for
the AssextZext + frame index pattern which was moderately
disgusting. The same could probably be accomplished
with a target specific node, but would still
require special handling of frame indexes.

[arsenm]

Add comment, move current handling into default implementation

[arsenm]

r317671

include/llvm/Target/TargetLowering.h

public:
   /// argument allows us to only collect the known bits that are shared by the
   /// requested vector elements.
   virtual void computeKnownBitsForTargetNode(const SDValue Op,
                                              KnownBits &Known,
                                              const APInt &DemandedElts,
                                              const SelectionDAG &DAG,
                                              unsigned Depth = 0) const;
 
+  virtual void computeKnownBitsForFrameIndex(const SDValue Op,
+                                             KnownBits &Known,
+                                             const APInt &DemandedElts,
+                                             const SelectionDAG &DAG,
+                                             unsigned Depth = 0) const {
+    return;
+  }
+
   /// This method can be implemented by targets that want to expose additional
   /// information about sign bits to the DAG Combiner. The DemandedElts
   /// argument allows us to only collect the minimum sign bits that are shared
   /// by the requested vector elements.
   virtual unsigned ComputeNumSignBitsForTargetNode(SDValue Op,
                                                    const APInt &DemandedElts,
                                                    const SelectionDAG &DAG,
                                                    unsigned Depth = 0) const;

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

case ISD::SMAX: {
     Known.One &= Known2.One;
     break;
   }
   case ISD::FrameIndex:
   case ISD::TargetFrameIndex:
     if (unsigned Align = InferPtrAlignment(Op)) {
       // The low bits are known zero if the pointer is aligned.
       Known.Zero.setLowBits(Log2_32(Align));
-      break;
     }
+
+    TLI->computeKnownBitsForFrameIndex(Op, Known, DemandedElts, *this, Depth);
     break;
 
   default:
     if (Opcode < ISD::BUILTIN_OP_END)
       break;
     LLVM_FALLTHROUGH;
   case ISD::INTRINSIC_WO_CHAIN:
   case ISD::INTRINSIC_W_CHAIN: