[Attributor][FIX] Reachability needs to account for readonly callees

The oversight caused us to ignore call sites that are effectively dead
when we computed reachability (or more precise the call edges of a
function). The problem is that loads in the readonly callee might depend
on stores prior to the callee. If we do not track the call edge we
mistakenly assumed the store before the call cannot reach the load.
The problem is nicely visible in:
  `llvm/test/Transforms/Attributor/ArgumentPromotion/basictest.ll`

Caused by D118673.

Fixes https://github.com/llvm/llvm-project/issues/53726