SelectionDAG: Add KnownBits and SignBits computation for EXTRACT_ELEMENT

v2: use getZExtValue
    add missing break
    codestyle

v3: add few more comments

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Matt Arsenault <Matthew.Arsenault@amd.com>
llvm-svn: 226880