better comment

Author: OCHyams

llvm/test/CodeGen/ARM/branch-folder-single-bb-crash.mir

@@ -1,8 +1,14 @@
 # NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
 # RUN: llc -mtriple=armv7-apple-ios --run-pass=branch-folder %s -o - | FileCheck %s
 
-## Don't crash in branch-folder's HoistCommonCodeInSuccs when TBB and FBB are
-## the same block.
+## We hoist code from successor blocks, and in abnormal circumstances we can
+## have control flow that branches to the same block. Test we handle that
+## correctly.
+##
+## bb.5 (empty fall-through) folds into bb.6, making bb.2 branch to bb.6
+## unconditionally on a conditional branch (both edges go to bb.6).
+## Instructions are hosited from bb.2's successors, which is bb.6 twice,
+## along both edges.
 
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