[X86] Remove unnecessary extra encodings from the CPU name enum in clang

Summary:
For a lot of older CPUs we have a 1:1 mapping between CPU name and enum name. But many of them are effectively aliases of each other and as a result are always repeated together at every usage

This patch removes most of the duplication. It also uses StringSwitch::Cases to make the many to one mapping in the StringSwitch more obvious.

Reviewers: RKSimon, spatel, zvi, igorb

Reviewed By: RKSimon

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37938

llvm-svn: 313462