[llvm] LSV: document hang reported in #37865 (PR #102479)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-win` running on `sie-win-worker` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/2973

Here is the relevant piece of the build log for the reference:
```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: Transforms/LoadStoreVectorizer/AArch64/pr37865.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 2
not --crash z:\b\llvm-clang-x86_64-sie-win\build\bin\opt.exe -mtriple=aarch64 -passes=load-store-vectorizer    -disable-output Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\Transforms\LoadStoreVectorizer\AArch64\pr37865.ll 2>&1 | z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\Transforms\LoadStoreVectorizer\AArch64\pr37865.ll
# executed command: not --crash 'z:\b\llvm-clang-x86_64-sie-win\build\bin\opt.exe' -mtriple=aarch64 -passes=load-store-vectorizer -disable-output 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\Transforms\LoadStoreVectorizer\AArch64\pr37865.ll'
# note: command had no output on stdout or stderr
# error: command failed with exit status: 1
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\Transforms\LoadStoreVectorizer\AArch64\pr37865.ll'
# .---command stderr------------
# | Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\Transforms\LoadStoreVectorizer\AArch64\pr37865.ll:6:10: error: CHECK: expected string not found in input
# | ; CHECK: Unexpected cycle while re-ordering instructions
# |          ^
# | <stdin>:1:1: note: scanning from here
# | z:\b\llvm-clang-x86_64-sie-win\build\bin\opt.exe: warning: failed to infer data layout: unable to get target for 'aarch64', see --version and --triple.
# | ^
# | <stdin>:2:44: note: possible intended match here
# | z:\b\llvm-clang-x86_64-sie-win\build\bin\opt.exe: WARNING: failed to create target machine for 'aarch64': unable to get target for 'aarch64', see --version and --triple.
# |                                            ^
# | 
# | Input file: <stdin>
# | Check file: Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\Transforms\LoadStoreVectorizer\AArch64\pr37865.ll
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |            1: z:\b\llvm-clang-x86_64-sie-win\build\bin\opt.exe: warning: failed to infer data layout: unable to get target for 'aarch64', see --version and --triple. 
# | check:6'0     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
# |            2: z:\b\llvm-clang-x86_64-sie-win\build\bin\opt.exe: WARNING: failed to create target machine for 'aarch64': unable to get target for 'aarch64', see --version and --triple. 
# | check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# | check:6'1                                                ?                                                                                                                               possible intended match
# | >>>>>>
# `-----------------------------
# error: command failed with exit status: 1

--

********************


```

https://github.com/llvm/llvm-project/pull/102479