[NFC] DeadArgumentElimination: Compute callers instead of Call origins

[kripken]

We had the list of Calls to each function, and used a map of Calls to
their origin functions to find all the callers of a function. Instead, just
track the set of callers directly, which avoids duplication and the extra
lookup table entry per Call (there can be many Calls from a source to a
target).

This makes a Dart testcase I am looking at run DAE 10% faster, and
DAE is the slowest pass by far.