[llvm] [SelectionDAG] Folding ZERO-EXTEND/SIGN_EXTEND poison to Poison value in getNode (PR #122741)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `premerge-monolithic-windows` running on `premerge-windows-1` while building `llvm` at step 8 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/35/builds/9384

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

```
Step 8 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/X86/bfloat.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 2
c:\ws\buildbot\premerge-monolithic-windows\build\bin\llc.exe < C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll -mtriple=i686-linux-gnu -mattr=avx512bf16,avx512fp16,avx512vl | c:\ws\buildbot\premerge-monolithic-windows\build\bin\filecheck.exe C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll --check-prefixes=X86
# executed command: 'c:\ws\buildbot\premerge-monolithic-windows\build\bin\llc.exe' -mtriple=i686-linux-gnu -mattr=avx512bf16,avx512fp16,avx512vl
# executed command: 'c:\ws\buildbot\premerge-monolithic-windows\build\bin\filecheck.exe' 'C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll' --check-prefixes=X86
# RUN: at line 3
c:\ws\buildbot\premerge-monolithic-windows\build\bin\llc.exe < C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll -mtriple=x86_64-linux-gnu | c:\ws\buildbot\premerge-monolithic-windows\build\bin\filecheck.exe C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll --check-prefixes=CHECK,SSE2
# executed command: 'c:\ws\buildbot\premerge-monolithic-windows\build\bin\llc.exe' -mtriple=x86_64-linux-gnu
# executed command: 'c:\ws\buildbot\premerge-monolithic-windows\build\bin\filecheck.exe' 'C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll' --check-prefixes=CHECK,SSE2
# .---command stderr------------
# | C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll:845:14: error: SSE2-NEXT: expected string not found in input
# | ; SSE2-NEXT: xorl %eax, %eax
# |              ^
# | <stdin>:404:9: note: scanning from here
# | # %bb.0:
# |         ^
# | <stdin>:438:2: note: possible intended match here
# |  orl %ebx, %eax
# |  ^
# | 
# | Input file: <stdin>
# | Check file: C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\X86\bfloat.ll
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |             .
# |             .
# |             .
# |           399:  .text 
# |           400:  .globl pr63017_2 
# |           401:  .p2align 4 
# |           402:  .type pr63017_2, at function 
# |           403: pr63017_2: # @pr63017_2 
# |           404: # %bb.0: 
# | next:845'0             X error: no match found
# |           405:  testb %al, %al 
# | next:845'0     ~~~~~~~~~~~~~~~~
# |           406:  jne .LBB16_1 
# | next:845'0     ~~~~~~~~~~~~~~
# |           407: # %bb.2: # %cond.load 
# | next:845'0     ~~~~~~~~~~~~~~~~~~~~~~
# |           408:  movzwl (%rax), %eax 
# | next:845'0     ~~~~~~~~~~~~~~~~~~~~~
# |           409:  shll $16, %eax 
...

```

</details>

https://github.com/llvm/llvm-project/pull/122741