[AVR] Avoid reusing the same variable name (NFC)

Apparently GCC 5.4 (a supported compiler) has a bug where it will
use the "MachineInstr &MI" defined by the range-based for loop
to evaluate the for loop expression. Pick a different variable
name to avoid this.

(cherry picked from commit f8f55f7)

llvm/lib/Target/AVR/AVRFrameLowering.cpp

@@ -298,11 +298,11 @@ bool AVRFrameLowering::restoreCalleeSavedRegisters(
 /// Replace pseudo store instructions that pass arguments through the stack with
 /// real instructions.
 static void fixStackStores(MachineBasicBlock &MBB,
-                           MachineBasicBlock::iterator MI,
+                           MachineBasicBlock::iterator StartMI,
                            const TargetInstrInfo &TII, Register FP) {
   // Iterate through the BB until we hit a call instruction or we reach the end.
   for (MachineInstr &MI :
-       llvm::make_early_inc_range(llvm::make_range(MI, MBB.end()))) {
+       llvm::make_early_inc_range(llvm::make_range(StartMI, MBB.end()))) {
     if (MI.isCall())
       break;