Revert "CodeGen: Use references in MachineTraceMetrics::Trace, NFC"

This reverts commit r261509.  I'm not sure how this compiled locally,
but something was out of whack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261510 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed