[SelectionDAG] Prevent warnings when extracting fixed length vector from scalable.

ComputeNumSignBits and computeKnownBits both trigger "Scalable flag
may be dropped" warnings when a fixed length vector is extracted
from a scalable vector.  This patch assumes nothing about the
demanded elements thus matching the behaviour when extracting a
scalable vector from a scalable vector.

Differential Revision: https://reviews.llvm.org/D83642