[MCA] Avoid an InstrDesc copy in mca::LSUnit::reserve.

Summary:
InstrDesc contains 4 vectors (as well as some other data), so it's
expensive to copy.

Authored By: orodley

Reviewers: andreadb, mattd, dberris

Reviewed By: mattd, dberris

Subscribers: dberris, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D49775

llvm-svn: 337985