AArch64: Make getNextNode() cleanup in r249764 more clear

After r249764, if you didn't see the full context, it looked like
`std::next(I)` would get the same result as
`++MachineBasicBlock::iterator(I)`.  However, `I` is a `MachineInstr*`
(not a `MachineBasicBlock::iterator`).

Use the `getIterator()` helper I added later (r249782) to make this code
more clear.

llvm-svn: 249852

llvm/lib/Target/AArch64/AArch64CleanupLocalDynamicTLSPass.cpp

@@ -118,8 +118,8 @@ struct LDTLSCleanup : public MachineFunctionPass {
 
     // Insert a copy from X0 to TLSBaseAddrReg for later.
     MachineInstr *Copy =
-        BuildMI(*I->getParent(), ++MachineBasicBlock::iterator(I),
-                I->getDebugLoc(), TII->get(TargetOpcode::COPY), *TLSBaseAddrReg)
+        BuildMI(*I->getParent(), ++I->getIterator(), I->getDebugLoc(),
+                TII->get(TargetOpcode::COPY), *TLSBaseAddrReg)
             .addReg(AArch64::X0);
 
     return Copy;