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.