[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-clang-x86_64-sie-win` running on `sie-win-worker` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/8443

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

```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: MachineVerifier/RISCV/subreg-liveness.mir' FAILED ********************
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 2
z:\b\llvm-clang-x86_64-sie-win\build\bin\llc.exe -mtriple=riscv64 -mattr=+v -run-pass=none Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\MachineVerifier\RISCV\subreg-liveness.mir -o - | z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\MachineVerifier\RISCV\subreg-liveness.mir
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\llc.exe' -mtriple=riscv64 -mattr=+v -run-pass=none 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\MachineVerifier\RISCV\subreg-liveness.mir' -o -
# .---command stderr------------
# | z:\b\llvm-clang-x86_64-sie-win\build\bin\llc.exe: error: unable to get target for 'riscv64', see --version and --triple.
# `-----------------------------
# error: command failed with exit status: 1
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\MachineVerifier\RISCV\subreg-liveness.mir'
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\MachineVerifier\RISCV\subreg-liveness.mir
# `-----------------------------
# error: command failed with exit status: 2

--

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


```

</details>

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