[SelectionDAG] Strengthen assertions about usage of AtomicSDNodes

[reames]

Summary

In D57601, I described the expectations around usage of AtomicSDNode and LoadSDNode/StoreSDNode w.r.t. atomic and volatiles. This patch simply embeds those expectations in assertions so that they can't be accidentally weakened.

Note: The reason only AtomicSDNodes of ATOMIC_LOAD and ATOMIC_STORE opcode are currently checked is that AMDGPU has an intrinsic which gets lowered to an ATOMIC_LOAD_FADD w/o a corresponding atomic MMO. This is suspicious, and I've brought it to the attention of the relevant developers. Once resolved, I'll strengthen that assertion.

Note 2: If this breaks your out-of-tree backend, go read the update instructions associated w/the previously mentioned patch. This will assert on (a subset of) things you need to update per those instructions.

[reames]

I somehow uploaded entirely the wrong patch... oops.

lib/CodeGen/AtomicExpandPass.cpp

bool AtomicExpand::expandAtomicOpToLibcall(
 
   // 'size' argument.
   if (!UseSizedLibcall) {
     // Note, getIntPtrType is assumed equivalent to size_t.
     Args.push_back(ConstantInt::get(DL.getIntPtrType(Ctx), Size));
   }
 
   // 'ptr' argument.
-  Value *PtrVal =
-      Builder.CreateBitCast(PointerOperand, Type::getInt8PtrTy(Ctx));
+  // note: This assumes all address spaces share a common libfunc
+  // implementation and that addresses are convertable.  For systems without
+  // that property, we'd need to extend this mechanism to support As-specific
+  // families of atomic intrinsics.
+  auto *PtrTypeAS0 = PointerOperand->getType()->getPointerTo(0);
+  Value *PtrVal = Builder.CreateAddrSpaceCast(PointerOperand, PtrTypeAS0);
+  PtrVal = Builder.CreateBitCast(PtrVal, Type::getInt8PtrTy(Ctx));
   Args.push_back(PtrVal);
 
   // 'expected' argument, if present.
   if (CASExpected) {
     AllocaCASExpected = AllocaBuilder.CreateAlloca(CASExpected->getType());
     AllocaCASExpected->setAlignment(AllocaAlignment);
     AllocaCASExpected_i8 =
         Builder.CreateBitCast(AllocaCASExpected, Type::getInt8PtrTy(Ctx));