CodeGen: Use MachineInstr& more in MachineTraceMetrics, NFC

Push MachineInstr& through helper APIs for consistency.  This doesn't
remove any more implicit conversions, but it's a nice cleanup after
r274300.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274301 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed