[llvm] RegisterCoalescer: Set undef on full register uses when coalescing implicit_def (PR #118321)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-offload-sles-build-only` running on `rocm-worker-hw-04-sles` while building `llvm` at step 8 "Add check check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/140/builds/12110

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

```
Step 8 (Add check check-llvm) failure: 1200 seconds without output running [b'ninja', b'-j 32', b'check-llvm'], attempting to kill
...
[681/685] Linking CXX executable unittests/Transforms/Scalar/ScalarTests
[682/685] Linking CXX executable unittests/Target/TargetMachineCTests
[683/685] Linking CXX executable unittests/tools/llvm-exegesis/LLVMExegesisTests
[684/685] Linking CXX executable unittests/Target/AMDGPU/AMDGPUTests
[684/685] Running the LLVM regression tests
llvm-lit: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/utils/lit/lit/llvm/config.py:506: note: using ld.lld: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/ld.lld
llvm-lit: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/utils/lit/lit/llvm/config.py:506: note: using lld-link: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/lld-link
llvm-lit: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/utils/lit/lit/llvm/config.py:506: note: using ld64.lld: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/ld64.lld
llvm-lit: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/utils/lit/lit/llvm/config.py:506: note: using wasm-ld: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/wasm-ld
-- Testing: 56373 tests, 32 workers --
command timed out: 1200 seconds without output running [b'ninja', b'-j 32', b'check-llvm'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1288.887750
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..

```

</details>

https://github.com/llvm/llvm-project/pull/118321