Minor latent bug fix in findGuaranteedReferenceRoots.
Minor latent bug fix in findGuaranteedReferenceRoots.

Fix an obvious mistake that happens to not break anything in practice.

Note that the argument index of a terminator result is not the same as
an operand index for the terminator instruction.

apple/swift,gregomni/swift,benlangmuir/swift,rudkx/swift,roambotics/swift,roambotics/swift,roambotics/swift,gregomni/swift,benlangmuir/swift,atrick/swift,atrick/swift,JGiola/swift,benlangmuir/swift,rudkx/swift,apple/swift,roambotics/swift,JGiola/swift,apple/swift,rudkx/swift,glessard/swift,gregomni/swift,roambotics/swift,JGiola/swift,JGiola/swift,rudkx/swift,glessard/swift,atrick/swift,gregomni/swift,ahoppen/swift,apple/swift,roambotics/swift,ahoppen/swift,apple/swift,glessard/swift,glessard/swift,gregomni/swift,rudkx/swift,ahoppen/swift,atrick/swift,rudkx/swift,atrick/swift,ahoppen/swift,ahoppen/swift,atrick/swift,JGiola/swift,ahoppen/swift,JGiola/swift,gregomni/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,apple/swift,glessard/swift