InstructionSimplify: Relanding r301766

Summary:
Re-applying r301766 with a fix to a typo and a regression test.

The log message for r301766 was:
==================================================================================
    InstructionSimplify: Canonicalize shuffle operands. NFC-ish.

    Summary:
     Apply canonicalization rules:
        1. Input vectors with no elements selected from can be replaced with undef.
        2. If only one input vector is constant it shall be the second one.

    This allows constant-folding to cover more ad-hoc simplifications that
    were in place and avoid duplication for RHS and LHS checks.

    There are more rules we may want to add in the future when we see a
    justification. e.g. mask elements that select undef elements can be
    replaced with undef.
==================================================================================

Reviewers: spatel, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32863

llvm-svn: 302373