[SLPVectorizer] Provide InstructionsState down the BoUpSLP vectorization call tree

As described in D48359, this patch pushes InstructionsState down the BoUpSLP call hierarchy instead of the corresponding raw OpValue. This makes it easier to track the alternate opcode etc. and avoids us having to call getAltOpcode which makes it difficult to support more than one alternate opcode.

Differential Revision: https://reviews.llvm.org/D48382

llvm-svn: 335170