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

[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.

llvm-svn: 339885