[GlobalISel] Start using vectors in GISelKnownBits

For vectors we consider a bit as known if it is the same for all demanded
vector elements (all elements by default). KnownBits BitWidth for vector
type is size of vector element. Add support for G_BUILD_VECTOR.
This allows combines of urem_pow2_to_mask in pre-legalizer combiner.

Differential Revision: https://reviews.llvm.org/D96122