[x86] Add a flag to control the vector shuffle legality predicates that

[x86] Add a flag to control the vector shuffle legality predicates that
complements the new vector shuffle lowering code path. This flag,
naturally, is *off* because we've not tested or evaluated the results of
this at all. However, the flag will make it much easier to evaluate
whether we can be this aggressive and whether there are missing vector
shuffle lowering optimizations.

llvm-svn: 225491