[llvm] [AMDGPU][SIPreEmitPeephole] pre-commit tests: mustRetainExeczBranch: use a cost model (PR #109816)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `mlir-rocm-mi200` running on `mi200-buildbot` while building `llvm` at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/177/builds/5642

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

```
Step 2 (checkout) failure: update (failure)
git version 2.25.1
remote: git: 'lariat' is not a git command. See 'git --help'.
remote: aborting due to possible repository corruption on the remote side.
fatal: protocol error: bad pack header
remote: git: 'lariat' is not a git command. See 'git --help'.        
remote: aborting due to possible repository corruption on the remote side.
fatal: protocol error: bad pack header

```

</details>

https://github.com/llvm/llvm-project/pull/109816