[SCEV] Make sure starting block is marked as visited when recursively collecting loop guards. (llvm#120749)

When `collectFromBlock` is called without a predecessor (in particular
for loops that don't have a unique predecessor outside the loop) we
never start climbing the predecessor chain, and thus don't mark the
starting block as visited.

Fixes llvm#120615.

Author: juliannagele

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -15765,6 +15765,7 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
   // original header.
   // TODO: share this logic with isLoopEntryGuardedByCond.
   unsigned NumCollectedConditions = 0;
+  VisitedBlocks.insert(Block);
   std::pair<const BasicBlock *, const BasicBlock *> Pair(Pred, Block);
   for (; Pair.first;
        Pair = SE.getPredecessorWithUniqueSuccessorForBB(Pair.first)) {

llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll

@@ -310,3 +310,29 @@ inner.header:
 exit:
   ret void
 }
+
+; Checks correct traversal for loops without a unique predecessor
+; outside the loop.
+define void @pr120615() {
+; CHECK-LABEL: pr120615
+; CHECK-NEXT:  Determining loop execution counts for: @pr120615
+; CHECK-NEXT:  Loop %header: backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %header: constant max backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %header: symbolic max backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %header: Trip multiple is 1
+entry:
+  br label %header
+
+bb:
+  br label %header
+
+header:
+  %0 = phi i32 [ %1, %header ], [ 0, %bb ], [ 0, %entry ]
+  %1 = add i32 %0, 1
+  %icmp = icmp slt i32 %0, 0
+  br i1 %icmp, label %header, label %exit
+
+exit:
+  ret void
+
+}