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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220345 91177308-0d34-0410-b5e6-96231b3b80d8