CallSiteInfo is originally used only for argument - register pairs. Make
it struct, in which we can store additional data for call sites.
Also, the variables/methods used for CallSiteInfo are named for its
original use case, e.g., CallFwdRegsInfo. Refactor these for the upcoming
use, e.g. addCallArgsForwardingRegs() -> addCallSiteInfo().
An upcoming patch will add type ids for indirect calls to propogate them from
middle-end to the back-end. The type ids will be then used to emit the call