[llvm-mca] PR39261: Rename FetchStage to EntryStage.

This fixes PR39261.

FetchStage is a misnomer. It causes confusion with the frontend fetch stage,
which we don't currently simulate.  I decided to rename it into EntryStage
mainly because this is meant to be a "source" stage for all pipelines.

Differential Revision: https://reviews.llvm.org/D54268

llvm-svn: 346419