[llvm] [LV] Add initial legality checks for ee loops with stores (PR #145663)

[Florian Hahn via llvm-commits]

================
@@ -515,20 +523,87 @@ class LoopVectorizationLegality {
   /// Returns true if this is an early exit loop that can be vectorized.
   /// Currently, a loop with an uncountable early exit is considered
   /// vectorizable if:
-  ///   1. There are no writes to memory in the loop.
+  ///   1. Writes to memory do not form a dependence with any load used as
+  ///      part of the uncounted exit condition.
   ///   2. The loop has only one early uncountable exit
   ///   3. The early exit block dominates the latch block.
   ///   4. The latch block has an exact exit count.
   ///   5. The loop does not contain reductions or recurrences.
   ///   6. We can prove at compile-time that loops will not contain faulting
-  ///   loads.
+  ///      loads, or that any faulting loads would also occur in a purely
+  ///      scalar loop.
   ///   7. It is safe to speculatively execute instructions such as divide or
-  ///   call instructions.
+  ///      call instructions.
   /// The list above is not based on theoretical limitations of vectorization,
   /// but simply a statement that more work is needed to support these
   /// additional cases safely.
   bool isVectorizableEarlyExitLoop();
 
+  /// When vectorizing an early exit loop containing side effects, we need to
+  /// determine whether an uncounted exit will be taken before any operation
----------------
fhahn wrote:

```suggestion
  /// determine whether an uncountable exit will be taken before any operation
```

https://github.com/llvm/llvm-project/pull/145663