CodeGen: Use MachineInstr& in PrologEpilogInserter, NFC

Use MachineInstr& over MachineInstr* to avoid implicit iterator to
pointer conversions.  MachineInstr*-as-nullptr was being used as a flag
for whether the for loop terminated normally; I added an explicit `bool`
instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274310 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed