[llvm] [InstCombine][NFC] Add test for existing fold (PR #181555)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve2-vla` running on `linaro-g4-01` while building `llvm` at step 14 "test-suite".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/198/builds/12040

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 14 (test-suite) failure: test (failure)
...
size..init_array: 16 
size..interp: 27 
size..note.ABI-tag: 32 
size..plt: 992 
size..rela.dyn: 3168 
size..rela.plt: 1440 
size..rodata: 27006 
size..text: 712096 
**********
NOEXE: test-suite :: Fortran/cray/f2023/16/09/tokenize01.test (1542 of 10356)
******************** TEST 'test-suite :: Fortran/cray/f2023/16/09/tokenize01.test' FAILED ********************
Executable '/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/test/sandbox/build/Fortran/cray/f2023/16/09/tokenize01' is missing
********************
PASS: test-suite :: Fortran/UnitTests/fcvs21_f95/FM832.test (1543 of 10356)
********** TEST 'test-suite :: Fortran/UnitTests/fcvs21_f95/FM832.test' RESULTS **********
compile_time: 0.0000 
exec_time: 0.0000 
hash: "2e8cd39e26808d2dd01324e03b827390" 
link_time: 0.0000 
size: 1143696 
size..bss: 336 
size..comment: 258 
size..data: 896 
size..data.rel.ro: 416 
size..dynamic: 496 
size..dynstr: 683 
size..dynsym: 1776 
size..eh_frame: 48056 
size..eh_frame_hdr: 12084 
size..fini: 20 
size..fini_array: 8 
size..gnu.hash: 28 
size..gnu.version: 148 
size..gnu.version_r: 96 
size..got: 392 
size..got.plt: 536 
size..init: 24 
size..init_array: 16 
size..interp: 27 
size..note.ABI-tag: 32 
size..plt: 1056 
size..rela.dyn: 3264 
size..rela.plt: 1536 
size..rodata: 27342 
size..text: 713056 
**********
PASS: test-suite :: Fortran/UnitTests/fcvs21_f95/FM825.test (1544 of 10356)
********** TEST 'test-suite :: Fortran/UnitTests/fcvs21_f95/FM825.test' RESULTS **********
compile_time: 0.0000 

```

</details>

https://github.com/llvm/llvm-project/pull/181555