[DAG] Teach computeKnownBits and ComputeNumSignBits in SelectionDAG to look through...

[DAG] Teach computeKnownBits and ComputeNumSignBits in SelectionDAG to look through EXTRACT_VECTOR_ELT.

Summary: Both computeKnownBits and ComputeNumSignBits can now do a simple
look-through of EXTRACT_VECTOR_ELT. It will compute the result based
on the known bits (or known sign bits) for the vector that the element
is extracted from.

Reviewers: bogner, tstellarAMD, mkuper

Subscribers: wdng, RKSimon, jyknight, llvm-commits, nhaehnle

Differential Revision: https://reviews.llvm.org/D25007

llvm-svn: 283347