MachineInstrBuilder: Introduce copyMIMetadata() function.
This reduces the amount of boilerplate required when adding a new
field to MIMetadata and reduces the chance of bugs like the
one I fixed in TargetInstrInfo::reassociateOps.
Reviewers: arsenm, nikic
Reviewed By: nikic
Pull Request: https://github.com/llvm/llvm-project/pull/133535
