Revert "CodeGen: MachineInstr::getIterator() => getInstrIterator(), NFC"

This reverts commit r261504, since it's not obvious the new name is
better:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160222/334298.html

I'll recommit if we get consensus that it's the right direction.

llvm-svn: 261567
diff --git a/llvm/lib/CodeGen/ProcessImplicitDefs.cpp b/llvm/lib/CodeGen/ProcessImplicitDefs.cpp
index ea5d234..d27ea2f 100644
--- a/llvm/lib/CodeGen/ProcessImplicitDefs.cpp
+++ b/llvm/lib/CodeGen/ProcessImplicitDefs.cpp
@@ -96,8 +96,8 @@
 
   // This is a physreg implicit-def.
   // Look for the first instruction to use or define an alias.
-  auto UserMI = MI->getInstrIterator();
-  auto UserE = MI->getParent()->instr_end();
+  MachineBasicBlock::instr_iterator UserMI = MI->getIterator();
+  MachineBasicBlock::instr_iterator UserE = MI->getParent()->instr_end();
   bool Found = false;
   for (++UserMI; UserMI != UserE; ++UserMI) {
     for (MachineOperand &MO : UserMI->operands()) {