CodeGen: Change MachineInstr to use MachineInstr&, NFC

Change MachineInstr API to prefer MachineInstr& over MachineInstr*
whenever the parameter is expected to be non-null.  Slowly inching
toward being able to fix PR26753.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262149 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed