[llvm-mca] Use a WriteRef to describe register writes in class RegisterFile.

This patch introduces a new class named WriteRef. A WriteRef is used by the
RegisterFile to keep track of register definitions. Internally it wraps a
WriteState, as well as the source index of the defining instruction.

This patch allows the tool to propagate additional information to support future
analysis on data dependencies.

llvm-svn: 335867