[llvm] [RISCV][MachineVerifier] Use RegUnit for register liveness checking (PR #115980)

[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 6 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/180/builds/8959

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

```
Step 6 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: MachineVerifier/RISCV/subreg-liveness.mir' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 2: /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc -mtriple=riscv64 -mattr=+v -run-pass=none /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/MachineVerifier/RISCV/subreg-liveness.mir -o - | /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/MachineVerifier/RISCV/subreg-liveness.mir
+ /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc -mtriple=riscv64 -mattr=+v -run-pass=none /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/MachineVerifier/RISCV/subreg-liveness.mir -o -
+ /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/MachineVerifier/RISCV/subreg-liveness.mir
/home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc: error: unable to get target for 'riscv64', see --version and --triple.FileCheck error: '<stdin>' is empty.
FileCheck command line:  /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/MachineVerifier/RISCV/subreg-liveness.mir

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/115980