[llvm-mca] Use a SmallVector instead of std::vector to track register reads/writes. NFCI

This avoids a heap allocation most of the times.
This patch gives a small but consistent 3% speedup on a release build (up to ~5%
on a debug build).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347464 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed