[X86] Return from SimplifyDemandedBitsForTargetNode after calculating known...

[X86] Return from SimplifyDemandedBitsForTargetNode after calculating known bits for VSHLI/VSRAI/VSRLI.

We were breaking out of the switch which falls into the default
implementation of SimplifyDemandedBitsForTargetNode which is a
wrapper around computeKnownBits. So we end up doing the recursion
and known bits calculation all over again. Instead we should return
with the known bits we calculated in the switch.