[ValueTracking] Add DemandedElts support to computeKnownBits/ComputeNumSignBits (PR36319)

This patch adds initial support for a DemandedElts mask to the internal computeKnownBits/ComputeNumSignBits methods, matching the SelectionDAG and GlobalISel equivalents.

So far only a couple of instructions have been setup to handle the DemandedElts, the remainder still using the existing 'all elements' default. The plan is to extend support as we have test coverage.

Differential Revision: https://reviews.llvm.org/D73435