[llvm] [SROA] Update the struct-to-vector test to be more representative (NFC) (PR #217498)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 180919 tests passed
* 3700 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### LLVM
<details>
<summary>LLVM.Transforms/SROA/struct-to-vector.ll</summary>

```
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 2
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/opt -passes='sroa<aggregate-to-vector>' -S /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/Transforms/SROA/struct-to-vector.ll | /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/Transforms/SROA/struct-to-vector.ll
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/opt '-passes=sroa<aggregate-to-vector>' -S /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/Transforms/SROA/struct-to-vector.ll
# .---command stderr------------
# | /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/opt: /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/Transforms/SROA/struct-to-vector.ll:54:1: error: expected top-level entity
# | SROA promotes the
# | ^
# `-----------------------------
# error: command failed with exit status: 1
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/Transforms/SROA/struct-to-vector.ll
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/Transforms/SROA/struct-to-vector.ll
# `-----------------------------
# error: command failed with exit status: 2

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/217498