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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315390 91177308-0d34-0410-b5e6-96231b3b80d8
20 files changed