[SCEV] Fix incorrect trip count computation

The way we elide max expressions when computing trip counts is incorrect
-- it breaks cases like this:

```
static int wrapping_add(int a, int b) {
  return (int)((unsigned)a + (unsigned)b);
}

void test() {
  volatile int end_buf = 2147483548; // INT_MIN - 100
  int end = end_buf;

  unsigned counter = 0;
  for (int start = wrapping_add(end,  200); start < end; start++)
    counter++;

  print(counter);
}
```

Note: the `NoWrap` variable that was being tested has little to do with
the values flowing into the max expression; it is a property of the
induction variable.

test/Transforms/LoopUnroll/nsw-tripcount.ll was added to solely test
functionality I'm reverting in this change, so I've deleted the test
fully.

llvm-svn: 273079