[CodeGen] Fix assert in SelectionDAG::computeKnownBits

Fix SelectionDAG::computeKnownBits asserting when handling EXTRACT_SUBVECTOR
when zero extending the demanded elements mask if it is already as long as the
source vector.

Differential Revision: https://reviews.llvm.org/D49574


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339600 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed