[SelectionDAG] Dump the DAG after legalizing vector ops and after the second type legalization

Summary:
With -debug, we aren't dumping the DAG after legalizing vector ops. In particular, on X86 with AVX1 only, we don't dump the DAG after we split 256-bit integer ops into pairs of 128-bit ADDs since this occurs during vector legalization.

I'm only dumping if the legalize vector ops changes something since we don't print anything during legalize vector ops. So this dump shows up right after the first type-legalization dump happens. So if nothing changed this second dump is unnecessary.

Having said that though, I think we should probably fix legalize vector ops to log what its doing.

Reviewers: RKSimon, eli.friedman, spatel, arsenm, chandlerc

Reviewed By: RKSimon

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D29554

llvm-svn: 294711