[SLPVectorizer] Remove RawInstructionsData/getMainOpcode and merge into getSameOpcode

This is part of the work to cleanup use of 'alternate' ops so we can use the more general SK_Select shuffle type.

Only getSameOpcode calls getMainOpcode and much of the logic is repeated in both functions. This will require some reworking of D28907 but that patch has hit trouble and is unlikely to be completed anytime soon.

Differential Revision: https://reviews.llvm.org/D48120

llvm-svn: 334701