[DAGCombine] Simplify funnel shifts with undef/zero args to bitshifts

Now that we have SimplifyDemandedBits support for funnel shifts (rL353539), we need to simplify funnel shifts back to bitshifts in cases where either argument has been folded to undef/zero.

Differential Revision: https://reviews.llvm.org/D58009

llvm-svn: 353645