[DAGCombiner] Extending pattern detection for vector shuffle.

    Summary:
    If all the operands of a BUILD_VECTOR extract elements from same vector then split the
    vector efficiently based on the maximum vector access index.

    Reviewers: zvi, delena, RKSimon, thakis

    Reviewed By: RKSimon

    Subscribers: chandlerc, eladcohen, llvm-commits

    Differential Revision: https://reviews.llvm.org/D35788

llvm-svn: 311255