[MCA] Remove dependency on CodeGen.

Summary:
There isn't any actual dependency - there's one #include from CodeGen
but nothing from the header is actually used.

With this change we can use the MCA library from CodeGen without
circular dependencies (e.g. for scheduling).

Reviewers: andreadb

Reviewed By: andreadb

Authored By: orodley

Subscribers: mgorny, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D52288

llvm-svn: 342706