[SLP] Look-ahead operand reordering heuristic.

This patch introduces a new heuristic for guiding operand reordering. The new "look-ahead" heuristic can look beyond the immediate predecessors. This helps break ties when the immediate predecessors have identical opcodes (see lit test for an example).

Committed on behalf of @vporpo (Vasileios Porpodas)

Differential Revision: https://reviews.llvm.org/D60897

llvm-svn: 364084