[llvm] [MIR] Serialize virtual register flags (PR #110228)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-gcc-ubuntu` running on `sie-linux-worker3` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/174/builds/6753

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/ARM/GlobalISel/arm-regbankselect.mir' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/llc -debugify-and-strip-all-safe=0 -mtriple arm-- -run-pass=regbankselect /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir -o - | /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/llc -debugify-and-strip-all-safe=0 -mtriple arm-- -run-pass=regbankselect /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir -o -
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir:112:10: error: CHECK: expected string not found in input
# CHECK: - { id: 0, class: gprb, preferred-register: '' }
         ^
<stdin>:266:11: note: scanning from here
registers:
          ^
<stdin>:267:2: note: possible intended match here
 - { id: 0, class: gprb, preferred-register: '', flags: [ ] }
 ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir:138:10: error: CHECK: expected string not found in input
# CHECK: - { id: 0, class: gprb, preferred-register: '' }
         ^
<stdin>:335:11: note: scanning from here
registers:
          ^
<stdin>:336:2: note: possible intended match here
 - { id: 0, class: gprb, preferred-register: '', flags: [ ] }
 ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir:164:10: error: CHECK: expected string not found in input
# CHECK: - { id: 0, class: gprb, preferred-register: '' }
         ^
<stdin>:404:11: note: scanning from here
registers:
          ^
<stdin>:405:2: note: possible intended match here
 - { id: 0, class: gprb, preferred-register: '', flags: [ ] }
 ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir:190:10: error: CHECK: expected string not found in input
# CHECK: - { id: 0, class: gprb, preferred-register: '' }
         ^
<stdin>:473:11: note: scanning from here
registers:
          ^
<stdin>:474:2: note: possible intended match here
 - { id: 0, class: gprb, preferred-register: '', flags: [ ] }
 ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/CodeGen/ARM/GlobalISel/arm-regbankselect.mir:216:10: error: CHECK: expected string not found in input
# CHECK: - { id: 0, class: gprb, preferred-register: '' }
         ^
<stdin>:542:11: note: scanning from here
registers:
          ^
...

```

</details>

https://github.com/llvm/llvm-project/pull/110228