[BasicBlockUtils] Add corner case test for loop metadata.

Adds a test to expose #55416.

Differential Revision: https://reviews.llvm.org/D125696

llvm/test/Transforms/LoopSimplify/update_latch_md2.ll

@@ -0,0 +1,36 @@
+; Tests loop-simplify does not move the loop metadata, because
+; the loopexit block is not the latch of the loop _bb6.
+
+; FIXME(#55416): The metadata should not move.
+
+; RUN: opt < %s -passes=loop-simplify -S | FileCheck %s
+; CHECK-LABEL: loop.header.loopexit:
+; CHECK: br label %loop.header, !llvm.loop !0
+; CHECK-LABEL: loop.latch:
+; CHECK-NOT: br i1 %p, label %loop.latch, label %loop.header.loopexit, !llvm.loop !0
+
+define void @func(i1 %p) {
+entry:
+  br label %loop.header
+
+loop.header:
+  br i1 %p, label %bb1, label %exit
+
+bb1:
+  br i1 %p, label %bb2, label %bb3
+
+bb2:
+  br label %bb3
+
+bb3:
+  br label %loop.latch
+
+loop.latch:
+  br i1 %p, label %loop.latch, label %loop.header, !llvm.loop !0
+
+exit:
+  ret void
+}
+
+!0 = distinct !{!0, !1}
+!1 = !{!"llvm.loop.mustprogress"}