[SLPVectorizer] Support alternate opcodes in tryToVectorizeList

Enable tryToVectorizeList to support InstructionsState alternate opcode patterns at a root (build vector etc.) as well as further down the vectorization tree.

NOTE: This patch reduces some of the debug reporting if there are opcode mismatches - I can try to add it back if it proves a problem. But it could get rather messy trying to provide equivalent verbose debug strings via getSameOpcode etc.

Differential Revision: https://reviews.llvm.org/D48488

llvm-svn: 335364