[X86] Add isel patterns for X86VBroadcast with i16 truncates from i16->i64 zextload/extload.
We can form vpbroadcastw with a folded load.
We had patterns for i16->i32 zextload/extload, but nothing prevents
i64 from occuring.
I'd like to move this all to DAG combine to fix more cases, but
this is trivial fix to minimize test diffs when moving to a combine.