Add a new step to legalization to legalize vector math operations. This

will allow simplifying LegalizeDAG to eliminate type legalization.  (I 
have a patch to do that, but it's not quite finished; I'll commit it 
once it's finished and I've fixed any review comments for this patch.)  
See the comment at the beginning of 
lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp for more details on the
motivation for this patch.

llvm-svn: 72325