IndexCall is a simple wrapper around:
PointerUnion<CallsiteInfo *, AllocInfo *>
Now, because we don't have CastInfo for IndexCall, we would have to
use getBase like so:
dyn_cast_if_present<CallsiteInfo *>(Call.getBase())
This patch adds simplify_type<IndexCall>, which in turn enables
CastInfo for IndexCall, so we can drop getBase like so::
dyn_cast_if_present<CallsiteInfo *>(Call)