Revert "[DAGCombiner] Enable SimplifyDemandedBits vector support for TRUNCATE"

It caused "Vector shift amounts must be in the same as their first arg"
asserts in Chromium builds. See the code review for repro instructions.

> Add DemandedElts support inside the TRUNCATE analysis.
>
> Differential Revision: https://reviews.llvm.org/D56387

This reverts commit cad4275d.