[SelectionDAG] Add BUILD_VECTOR support to computeKnownBits and SimplifyDemandedBits

Add the ability to computeKnownBits and SimplifyDemandedBits to extract the known zero/one bits from BUILD_VECTOR, returning the known bits that are shared by every vector element.

This is an initial step towards determining the sign bits of a vector (PR29079).

Differential Revision: https://reviews.llvm.org/D24253

llvm-svn: 280927