LSV: document hang reported in #37865 (#102479)

LoadStoreVectorizer hangs on certain examples, when its reorder function
goes into a cycle. Detect this cycle and explicitly forbid it, using an
assert, and document the resulting crash in a test-case under AArch64.

llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp

@@ -216,6 +216,8 @@ void reorder(Instruction *I) {
       if (IM->getParent() != I->getParent())
         continue;
 
+      assert(IM != I && "Unexpected cycle while re-ordering instructions");
+
       if (!IM->comesBefore(I)) {
         InstructionsToMove.insert(IM);
         Worklist.push_back(IM);

llvm/test/Transforms/LoadStoreVectorizer/AArch64/pr37865.ll

@@ -0,0 +1,15 @@
+; REQUIRES: asserts
+; RUN: not --crash opt -mtriple=aarch64 -passes=load-store-vectorizer \
+; RUN:   -disable-output %s 2>&1 | FileCheck %s
+
+define i32 @load_cycle(ptr %x) {
+; CHECK: Unexpected cycle while re-ordering instructions
+entry:
+  %gep.x.1 = getelementptr inbounds [2 x i32], ptr %x, i32 0, i32 1
+  %load.x.1 = load i32, ptr %gep.x.1
+  %rem = urem i32 %load.x.1, 1
+  %gep.x.2 = getelementptr inbounds [2 x i32], ptr %x, i32 %rem, i32 0
+  %load.x.2 = load i32, ptr %gep.x.2
+  %ret = add i32 %load.x.2, %load.x.1
+  ret i32 %ret
+}