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, andreadb, davide

Reviewed By: spatel, RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32338

llvm-svn: 301766