LTO: respect command-line options that disable vectorization.

Summary: Patches 202051 and 208013 added calls to LTO's PassManager which unconditionally add LoopVectorizePass and SLPVectorizerPass instead of following the logic in PassManagerBuilder::populateModulePassManager and honoring the -vectorize-loops -run-slp-after-loop-vectorization flags.

Reviewers: nadav, aschwaighofer, yijiang

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5884