[SLP] Make HorizontalReduction::getOperationData take an Instruction* instead of a Value*. NFCI

All of the callers already have an Instruction *. Many of them
from a dyn_cast.

Also update the OperationData constructor to use a Instruction&
to remove a dyn_cast and make it clear that the pointer is non-null.

Differential Revision: https://reviews.llvm.org/D88132