[SLP]Try to emit canonical shuffle with undef operand.

In the canonical form of the shuffle the poison/undef operand is the
second operand, the patch tries to emit canonical form for partial
vectorization of the buildvector sequence.
Also, this patch starts emitting freeze instruction for shuffles with undef indices if the second shuffle operan is undef, not poison. It is an initial step to D93818, where undef mask element are treated as returning poison value.

Differential Revision: https://reviews.llvm.org/D134377