[llvm-mca] Small refactoring in preparation for another patch that will improve the modularity of the Pipeline. NFCI

The main difference is that now `cycleStart()` and `cycleEnd()` return an
llvm::Error.

This patch implements a few minor style changes, and adds missing 'const' to
some methods.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339885 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed