[AVX-512] Add support for commuting VPERMT2(B/W/D/Q/PS/PD) to/from VPERMI2(B/W/D/Q/PS/PD).

Summary:
The index and one of the table operands can be swapped by changing the opcode to the other version. Neither of these operands are the one that can load from memory so this can't be used to increase memory folding opportunities.

We need to handle the unmasked forms and the kz forms. Since the load operand isn't being commuted we can commute the load and broadcast instructions too.

Reviewers: igorb, delena, Ayal, Farhana, RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25652

llvm-svn: 287621