[SCEV] Support addrec in right hand side in howManyLessThans (#92560)

Fixes #92554 (std::reverse will auto-vectorize now)

When calculating number of times a exit condition containing a
comparison is executed, we mostly assume that RHS of comparison should
be loop invariant, but it may be another add-recurrence.

~In that case, we can try the computation with `LHS = LHS - RHS` and
`RHS = 0`.~ (It is not valid unless proven that it doesn't wrap)

**Edit:**
We can calculate back edge count for loop structure like:

```cpp
left = left_start
right = right_start
while(left < right){
  // ...do something...
  left += s1; // the stride of left is s1 (> 0)
  right -= s2; // the stride of right is -s2 (s2 > 0)
}
// left and right converge somewhere in the middle of their start values
```
We can calculate the backedge-count as ceil((End - left_start) /u (s1-
(-s2)) where, End = max(left_start, right_start).

**Alive2**: https://alive2.llvm.org/ce/z/ggxx58

0 files changed, 0 insertions, 0 deletions