[X86] Add prefer-128-bit subtarget feature.

Summary:
Similar to the previous prefer-256-bit flag. We might want to
enable this by default some CPUs. This just starts the initial
work to implement and prove that it effects TTI's vector width.

Reviewers: RKSimon, echristo, spatel, atdt

Reviewed By: RKSimon

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67311

llvm-svn: 371319