[GlobalISel] (fix) Use pointer type size for offset constant when lowering stores

Commit 9965b12fd1 was supposed to change the offset constant when
lowering load/stores, but only introduced this change for loads. This
patch adds the same fix for stores.

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -2346,8 +2346,8 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
 
     // Generate the PtrAdd and truncating stores.
     LLT PtrTy = MRI.getType(PtrReg);
-    auto OffsetCst =
-        MIRBuilder.buildConstant(LLT::scalar(64), LargeSplitSize / 8);
+    auto OffsetCst = MIRBuilder.buildConstant(
+            LLT::scalar(PtrTy.getSizeInBits()), LargeSplitSize / 8);
     Register PtrAddReg = MRI.createGenericVirtualRegister(PtrTy);
     auto SmallPtr =
         MIRBuilder.buildPtrAdd(PtrAddReg, PtrReg, OffsetCst.getReg(0));