[SLP] Refactoring of the operand reordering code.

This is a refactoring patch which should have all the functionality of the current code. Its goal is twofold:
i. Cleanup and simplify the reordering code, and
ii. Generalize reordering so that it will work for an arbitrary number of operands, not just 2.

This is the second patch in a series of patches that will enable operand reordering across chains of operations. An example of this was presented in EuroLLVM'18 https://www.youtube.com/watch?v=gIEn34LvyNo .

Committed on behalf of @vporpo (Vasileios Porpodas)

Differential Revision: https://reviews.llvm.org/D59973

llvm-svn: 358519