[DAGCombiner] Enable SimplifyDemandedBits vector support for TRUNCATE (REAPPLIED).

Add DemandedElts support inside the TRUNCATE analysis.

REAPPLIED - this was reverted by @hans at rGa5122605 due to an issue with vector shift amount types, which was fixed in rG935bacd3 and an additional test case added at rG0ca81b90

Differential Revision: https://reviews.llvm.org/D56387