[Attributor] Deduction based on path exploration

This patch introduces the propagation of known information based on path exploration.
For example,
```
int u(int c, int *p){
  if(c) {
     return *p;
  } else {
     return *p + 1;
  }
}
```
An argument `p` is dereferenced whatever c's value is.

For an instruction `CtxI`, we accumulate branch instructions in the must-be-executed-context of `CtxI` and then, we take the conjunction of the successors' known state.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D65593