[analyzer] Track the right hand side of the last store regardless of its value

Summary:
The following code snippet taken from D64271#1572188 has an issue: namely,
because `flag`'s value isn't undef or a concrete int, it isn't being tracked.

int flag;
bool coin();

void foo() {
  flag = coin();
}

void test() {
  int *x = 0;
  int local_flag;
  flag = 1;

  foo();
  local_flag = flag;
  if (local_flag)
    x = new int;

  foo();
  local_flag = flag;
  if (local_flag)
    *x = 5;
}

This, in my opinion, makes no sense, other values may be interesting too.
Originally added by rC185608.

Differential Revision: https://reviews.llvm.org/D64287

llvm-svn: 368773