[LoopAccesses] Make VectorizerParams global

As LAA is becoming a pass, we can no longer pass the params to its
constructor.  This changes the command line flags to have external
storage.  These can now be accessed both from LV and LAA.

VectorizerParams is moved out of LoopAccessInfo in order to shorten the
code to access it.

This is part of the patchset that converts LoopAccessAnalysis into an
actual analysis pass.

llvm-svn: 229622