[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 `premerge-monolithic-linux` running on `premerge-linux-1` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/11723

<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 :: CodeGen/PowerPC/aix-p8vector-liveins.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /build/buildbot/premerge-monolithic-linux/build/bin/llc -mtriple powerpc-ibm-aix-xcoff -mcpu=pwr8    -verify-machineinstrs -stop-after=finalize-isel  < /build/buildbot/premerge-monolithic-linux/llvm-project/llvm/test/CodeGen/PowerPC/aix-p8vector-liveins.ll |    /build/buildbot/premerge-monolithic-linux/build/bin/FileCheck --check-prefixes=POWR8,VSX /build/buildbot/premerge-monolithic-linux/llvm-project/llvm/test/CodeGen/PowerPC/aix-p8vector-liveins.ll
+ /build/buildbot/premerge-monolithic-linux/build/bin/llc -mtriple powerpc-ibm-aix-xcoff -mcpu=pwr8 -verify-machineinstrs -stop-after=finalize-isel
+ /build/buildbot/premerge-monolithic-linux/build/bin/FileCheck --check-prefixes=POWR8,VSX /build/buildbot/premerge-monolithic-linux/llvm-project/llvm/test/CodeGen/PowerPC/aix-p8vector-liveins.ll
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/test/CodeGen/PowerPC/aix-p8vector-liveins.ll:21:10: error: POWR8: expected string not found in input
; POWR8: - { id: 0, class: vssrc, preferred-register: '' }
         ^
<stdin>:29:11: note: scanning from here
name: vssr
          ^
<stdin>:52:2: note: possible intended match here
 - { id: 0, class: vssrc, preferred-register: '', flags: [ ] }
 ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/test/CodeGen/PowerPC/aix-p8vector-liveins.ll:74:8: error: VSX: expected string not found in input
; VSX: - { id: 0, class: vsfrc, preferred-register: '' }
       ^
<stdin>:136:11: note: scanning from here
registers:
          ^
<stdin>:137:2: note: possible intended match here
 - { id: 0, class: vsfrc, preferred-register: '', flags: [ ] }
 ^

Input file: <stdin>
Check file: /build/buildbot/premerge-monolithic-linux/llvm-project/llvm/test/CodeGen/PowerPC/aix-p8vector-liveins.ll

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

Input was:
<<<<<<
            .
            .
            .
           24:   
           25:  attributes #0 = { "target-cpu"="pwr8" } 
           26:  
           27: ... 
           28: --- 
           29: name: vssr 
check:21'0               X error: no match found
           30: alignment: 16 
check:21'0     ~~~~~~~~~~~~~~
           31: exposesReturnsTwice: false 
check:21'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
           32: legalized: false 
check:21'0     ~~~~~~~~~~~~~~~~~
...

```

</details>

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