CodeGen: Use MachineInstr& in LiveVariables API, NFC

Change all the methods in LiveVariables that expect non-null
MachineInstr* to take MachineInstr& and update the call sites.  This
clarifies the API, and designs away a class of iterator to pointer
implicit conversions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274319 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed