[Loads] Keep using constant max BTCs for loops w/o computable BTCs.

Follow-up to cad62df to preserve the existing behavior for early-exit
loops, to fix vec.stats.ll test failure.

Author: fhahn

llvm/lib/Analysis/Loads.cpp

@@ -333,6 +333,14 @@ bool llvm::isDereferenceableAndAlignedInLoop(
   if (isa<SCEVCouldNotCompute>(MaxBECount))
     return false;
 
+  if (isa<SCEVCouldNotCompute>(BECount)) {
+    // TODO: Support symbolic max backedge taken counts for loops without
+    // computable backedge taken counts.
+    MaxBECount =
+        Predicates
+            ? SE.getPredicatedConstantMaxBackedgeTakenCount(L, *Predicates)
+            : SE.getConstantMaxBackedgeTakenCount(L);
+  }
   const auto &[AccessStart, AccessEnd] = getStartAndEndForAccess(
       L, PtrScev, LI->getType(), BECount, MaxBECount, &SE, nullptr);
   if (isa<SCEVCouldNotCompute>(AccessStart) ||