[NFC] Change SelectionDAG::getMemBasePlusOffset() to use int64_t

[arichardson]

Summary

This change is preparatory work to use this helper functions in more places.
Currently the function only allows positive offsets, but there are cases
where we want to subtract an offset from an existing pointer.

The motivation for this change is our out-of-tree CHERI backend
(https://github.com/CTSRD-CHERI/llvm-project). We use a separate register
type to store pointers (128-bit capabilities, which are effectively
unforgeable and monotonic fat pointers). These capabilities permit a
reduced set of operations and therefore use a separate ValueType (iFATPTR).
to represent pointers implemented as capabilities.
Therefore, we need to avoid using ISD::ADD for our patterns that operate
on pointers and need to use a function that chooses ISD::ADD or a new
ISD::PTRADD opcode depending on the value type.

We originally added a new DAG.getPointerAdd() function, but after this
patch series we can modify the implementation of getMemBasePlusOffset()
instead. Avoiding direct uses of ISD::ADD for pointer types will
significantly reduce the amount of assertion/instruction selection
failures for us in future upstream merges.

[merge_guards_bot]

Build result: fail - 60625 tests passed, 1 failed and 726 were skipped.

failed: LLVM.CodeGen/RISCV/mir-target-flags.ll

Log files: console-log.txt, CMakeCache.txt

[spatel]

LGTM

llvm/include/llvm/CodeGen/SelectionDAG.h

getTruncStore(SDValue Chain, const SDLoc &dl, SDValue Val, SDValue Ptr,
                 MachineMemOperand::Flags MMOFlags = MachineMemOperand::MONone,
                 const AAMDNodes &AAInfo = AAMDNodes());
   SDValue getTruncStore(SDValue Chain, const SDLoc &dl, SDValue Val,
                         SDValue Ptr, EVT SVT, MachineMemOperand *MMO);
   SDValue getIndexedStore(SDValue OrigStore, const SDLoc &dl, SDValue Base,
                           SDValue Offset, ISD::MemIndexedMode AM);
 
   /// Returns sum of the base pointer and offset.
-  SDValue getMemBasePlusOffset(SDValue Base, unsigned Offset, const SDLoc &DL);
+  /// Unlike getObjectPtrOffset this does not set NoUnsignedWrap by default.
+  SDValue getMemBasePlusOffset(SDValue Base, int64_t Offset, const SDLoc &DL);
 
   SDValue getMaskedLoad(EVT VT, const SDLoc &dl, SDValue Chain, SDValue Base,
                         SDValue Offset, SDValue Mask, SDValue Src0, EVT MemVT,
                         MachineMemOperand *MMO, ISD::MemIndexedMode AM,
                         ISD::LoadExtType, bool IsExpanding = false);
   SDValue getIndexedMaskedLoad(SDValue OrigLoad, const SDLoc &dl, SDValue Base,
                                SDValue Offset, ISD::MemIndexedMode AM);
   SDValue getMaskedStore(SDValue Chain, const SDLoc &dl, SDValue Val,

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

static SDValue getMemsetStringVal(EVT VT, const SDLoc &dl, SelectionDAG &DAG,
   // If the "cost" of materializing the integer immediate is less than the cost
   // of a load, then it is cost effective to turn the load into the immediate.
   Type *Ty = VT.getTypeForEVT(*DAG.getContext());
   if (TLI.shouldConvertConstantLoadToIntImm(Val, Ty))
     return DAG.getConstant(Val, dl, VT);
   return SDValue(nullptr, 0);
 }
 
-SDValue SelectionDAG::getMemBasePlusOffset(SDValue Base, unsigned Offset,
+SDValue SelectionDAG::getMemBasePlusOffset(SDValue Base, int64_t Offset,
                                            const SDLoc &DL) {
   EVT VT = Base.getValueType();
   return getNode(ISD::ADD, DL, VT, Base, getConstant(Offset, DL, VT));
 }
 
 /// Returns true if memcpy source is constant data.
 static bool isMemSrcFromConstant(SDValue Src, ConstantDataArraySlice &Slice) {
   uint64_t SrcDelta = 0;