[InstCombine] Make InstCombiner::OptAndOp take a BinaryOperator instead of an Instruction.

The callers have already performed the necessary cast before calling. This allows us to remove a comment that says the instruction must be a BinaryOperator and make it explicit in the argument type.

Had to add a default case to the switch because BinaryOperator::getOpcode() returns a BinaryOps enum.

llvm-svn: 299339