[SLP] Refactoring of EdgeInfo and UserTreeIdx in buildTree_rec().

This is a follow-up refactoring patch after the introduction of usable TreeEntry pointers in D61706.
The EdgeInfo struct can now use a TreeEntry pointer instead of an index in VectorizableTree.

Committed on behalf of @vporpo (Vasileios Porpodas)

Differential Revision: https://reviews.llvm.org/D61795

llvm-svn: 361110