[llvm] PowerPC: Use REG_SEQUENCE instead of INSERT_SUBREG (PR #129941)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx-nvidia-ubuntu` running on `as-builder-7` while building `llvm` at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/180/builds/14922

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

```
Step 2 (checkout) failure: update (failure)
git version 2.34.1
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

```

</details>

https://github.com/llvm/llvm-project/pull/129941