[PM] Port SLPVectorizer to the new PM

This uses the "runImpl" approach to share code with the old PM.

Porting to the new PM meant abandoning the anonymous namespace enclosing
most of SLPVectorizer.cpp which is a bit of a bummer (but not a big deal
compared to having to pull the pass class into a header which the new PM
requires since it calls the constructor directly).

llvm-svn: 272766