[x86] try to form broadcast before widening shuffle elements

I noticed that we weren't generating broadcasts as much I thought we would with 
D54271, and this is part of the problem.

Widening the shuffle elements means adding bitcasts and hiding the relationship 
between a splatted scalar and the vector. If we can form a broadcast, do that 
before going through the rest of the shuffle lowering because broadcasts should 
be cheap and can often be load-folded.

Differential Revision: https://reviews.llvm.org/D54280

llvm-svn: 346498