[SLP] Cleanup: Remove `tryToVectorizePair()`, most probably NFC

`tryToVectorizePair()` adds a level of indirection over `tryToVectorizeList()`.
I am not really sure why it is needed, it looks redundant.

I replaced all calls to `tryToVectorizePair()` with calls to
`tryToVectorizeList()` and I am not seeing any failures.

Differential Revision: https://reviews.llvm.org/D151004