[SLPVectorizer] Support alternate opcodes in tryToVectorizeList

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335364 91177308-0d34-0410-b5e6-96231b3b80d8