FunctionPropertiesAnalysis: handle callsite BBs that lose edges

There could be successors that were reached before but now are only
reachable from elsewhere in the CFG.

Suppose the following diamond CFG (lines are arrows pointing down):
    A
  /   \
 B     C
  \   /
    D
There's a call site in C that is inlined. Upon doing that, it turns out
it expands to:
   call void @llvm.trap()
   unreachable
D isn't reachable from C anymore, but we did discount it when we set up
FunctionPropertiesUpdater, so we need to re-include it here.

The patch also updates loop accounting to use LoopInfo rather than
traverse BBs.

Differential Revision: https://reviews.llvm.org/D127353