CodeGen: Minor cleanups to use MachineInstr::getMF. NFC

Since r315388 we have a shorter way to say this, so we'll replace
MI->getParent()->getParent() with MI->getMF() in a few places.

llvm-svn: 315390
diff --git a/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp b/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
index 9d02422..2eab037 100644
--- a/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
+++ b/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
@@ -401,7 +401,7 @@
         continue;
       }
 
-      const MachineFunction *MF = MI.getParent()->getParent();
+      const MachineFunction *MF = MI.getMF();
       const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
       BaseReg = Fn.getRegInfo().createVirtualRegister(RC);