[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-x86_64-debian-dylib` running on `gribozavr4` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/10132

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

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /b/1/llvm-x86_64-debian-dylib/build/bin/llc -O0 -mtriple arm64-- -run-pass=regbankselect /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir -o - | /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir
+ /b/1/llvm-x86_64-debian-dylib/build/bin/llc -O0 -mtriple arm64-- -run-pass=regbankselect /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir -o -
+ /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir:34:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: - { id: 0, class: gpr, preferred-register: '' }
              ^
<stdin>:55:11: note: scanning from here
registers:
          ^
<stdin>:56:2: note: possible intended match here
 - { id: 0, class: gpr, preferred-register: '', flags: [ ] }
 ^

Input file: <stdin>
Check file: /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/GlobalISel/regbankselect-dbg-value.mir

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           .
           .
           .
          50: hasEHFunclets: false 
          51: isOutlined: false 
          52: debugInstrRef: false 
          53: failsVerification: false 
          54: tracksDebugUserValues: false 
          55: registers: 
next:34'0               X error: no match found
          56:  - { id: 0, class: gpr, preferred-register: '', flags: [ ] } 
next:34'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
next:34'1      ?                                                            possible intended match
          57: liveins: [] 
next:34'0     ~~~~~~~~~~~~
          58: frameInfo: 
next:34'0     ~~~~~~~~~~~
          59:  isFrameAddressTaken: false 
next:34'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          60:  isReturnAddressTaken: false 
next:34'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          61:  hasStackMap: false 
next:34'0     ~~~~~~~~~~~~~~~~~~~~
           .
           .
...

```

</details>

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