[PATCH] [X86] Skip concat_vectors when lowering vector broadcast

[Nadav Rotem]

> 
> Yes, that was my alternative (handle it in DAGCombiner::visitCONCAT_
> VECTORS).  My fear was that the general combine could get quite
> complex and as I'm new to LLVM I'd get stuck.  Would it be acceptable
> to add a simple combine that just handled the case above in the first
> instance?


Sure, lets start by handling the simple case first. :)