[VPlan] Skip disconnected exit blocks in hasEarlyExit. (#151718)

Currently hasEarlyExit returns true, if there are multiple exit blocks.
ExitBlocks contains the wrapped original IR exit blocks. Without
checking the predecessors we incorrectly return true for loops with
multiple countable exits, that have been vectorized by requiring a
scalar epilogue. In that case, the exit blocks will get disconnected.

Fix this by filtering out disconnected exit blocks.

Currently this should only impact the 'early-exit vectorized' statistic.

PR: llvm/llvm-project#151718

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -4228,7 +4228,10 @@ class VPlan {
   /// block with multiple predecessors (one for the exit via the latch and one
   /// via the other early exit).
   bool hasEarlyExit() const {
-    return ExitBlocks.size() > 1 ||
+    return count_if(ExitBlocks,
+                    [](VPIRBasicBlock *EB) {
+                      return EB->getNumPredecessors() != 0;
+                    }) > 1 ||
            (ExitBlocks.size() == 1 && ExitBlocks[0]->getNumPredecessors() > 1);
   }
 

llvm/test/Transforms/LoopVectorize/vect.stats.ll

@@ -5,7 +5,7 @@
 ; vectorized) and the third one is not.
 
 ; CHECK: 4 loop-vectorize               - Number of loops analyzed for vectorization
-; CHECK: 2 loop-vectorize               - Number of early exit loops vectorized
+; CHECK: 1 loop-vectorize               - Number of early exit loops vectorized
 ; CHECK: 3 loop-vectorize               - Number of loops vectorized
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"