NFC: The Cost Model specialization, by Andrey Tischenko

The current Cost Model implementation is very inaccurate and has to be
updated, improved, re-implemented to be able to take into account the
concrete CPU models and the concrete targets where this Cost Model is
being used. For example, the Latency Cost Model should be differ from
Code Size Cost Model, etc.
This patch is the first step to launch the developing and implementation
of a new Cost Model generation.

Differential Revision: https://reviews.llvm.org/D25186

llvm-svn: 284012