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/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index ea655e1..ec971e1 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -866,7 +866,7 @@
 TargetLoweringBase::emitPatchPoint(MachineInstr &InitialMI,
                                    MachineBasicBlock *MBB) const {
   MachineInstr *MI = &InitialMI;
-  MachineFunction &MF = *MI->getParent()->getParent();
+  MachineFunction &MF = *MI->getMF();
   MachineFrameInfo &MFI = MF.getFrameInfo();
 
   // We're handling multiple types of operands here: