Split SETCC if VSELECT requires splitting too.

This patch is a rewrite of the original patch commited in r194542. Instead of
relying on the type legalizer to do the splitting for us, we now peform the
splitting ourselves in the DAG combiner. This is necessary for the case where
the vector mask is a legal type after promotion and still wouldn't require
splitting.

Patch by: Juergen Ributzka

NOTE: This is a candidate for the 3.4 branch.
llvm-svn: 195397