[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-aarch64-darwin` running on `doug-worker-4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/7569

<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/AArch64/GlobalISel/arm64-regbankselect.mir' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc -O0 -debugify-and-strip-all-safe -run-pass=regbankselect /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir -o - -verify-machineinstrs | /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir --check-prefix=CHECK --check-prefix=FAST
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc -O0 -debugify-and-strip-all-safe -run-pass=regbankselect /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir -o - -verify-machineinstrs
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir --check-prefix=CHECK --check-prefix=FAST
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir:219:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: - { id: 0, class: gpr32, preferred-register: '' }
              ^
<stdin>:525:11: note: scanning from here
registers:
          ^
<stdin>:526:2: note: possible intended match here
 - { id: 0, class: gpr32, preferred-register: '', flags: [ ] }
 ^
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir:362:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: - { id: 0, class: gpr64, preferred-register: '' }
              ^
<stdin>:880:11: note: scanning from here
registers:
          ^
<stdin>:881:2: note: possible intended match here
 - { id: 0, class: gpr64, preferred-register: '', flags: [ ] }
 ^
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir:400:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: - { id: 0, class: gpr, preferred-register: '' }
              ^
<stdin>:1004:11: note: scanning from here
registers:
          ^
<stdin>:1005:2: note: possible intended match here
 - { id: 0, class: gpr, preferred-register: '', flags: [ ] }
 ^
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir:426:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: - { id: 0, class: fpr, preferred-register: '' }
              ^
<stdin>:1068:11: note: scanning from here
registers:
          ^
<stdin>:1069:2: note: possible intended match here
 - { id: 0, class: fpr, preferred-register: '', flags: [ ] }
 ^
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/arm64-regbankselect.mir:451:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: - { id: 0, class: gpr, preferred-register: '' }
              ^
<stdin>:1132:11: note: scanning from here
registers:
          ^
...

```

</details>

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