[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 `llvm-clang-x86_64-sie-win` running on `sie-win-worker` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/15515

<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/X86/avx512-i1test.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 2
z:\b\llvm-clang-x86_64-sie-win\build\bin\llc.exe < Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\CodeGen\X86\avx512-i1test.ll -mattr=+avx512f -disable-cgp-delete-phis | z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\CodeGen\X86\avx512-i1test.ll
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\llc.exe' -mattr=+avx512f -disable-cgp-delete-phis
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\CodeGen\X86\avx512-i1test.ll'
# .---command stderr------------
# | Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\CodeGen\X86\avx512-i1test.ll:11:15: error: CHECK-NEXT: expected string not found in input
# | ; CHECK-NEXT: xorl %eax, %eax
# |               ^
# | <stdin>:8:16: note: scanning from here
# | # %bb.0: # %bb1
# |                ^
# | <stdin>:9:3: note: possible intended match here
# |  testb %al, %al
# |   ^
# | 
# | Input file: <stdin>
# | Check file: Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\CodeGen\X86\avx512-i1test.ll
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |            1:  .file "<stdin>" 
# |            2:  .text 
# |            3:  .globl func # -- Begin function func 
# |            4:  .p2align 4 
# |            5:  .type func, at function 
# |            6: func: # @func 
# | label:9'0     ^~~~~
# | label:9'1     ^~~~~
# |            7:  .cfi_startproc 
# |            8: # %bb.0: # %bb1 
# | check:10      ^~~~~~~~~~~~~~~
# | next:11'0                    X error: no match found
# |            9:  testb %al, %al 
# | next:11'0     ~~~~~~~~~~~~~~~~
# | next:11'1       ?              possible intended match
# |           10:  je .LBB0_1 
# | next:11'0     ~~~~~~~~~~~~
# |           11: # %bb.3: # %L_30 
# | next:11'0     ~~~~~~~~~~~~~~~~~
# |           12:  retq 
# | next:11'0     ~~~~~~
# |           13:  .p2align 4 
# | next:11'0     ~~~~~~~~~~~~
...

```

</details>

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