[llvm] [DAGCombiner] Don't ignore N2's undef elements in `foldVSelectOfConstants` (PR #129272)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-aarch64-ubuntu` running on `linaro-lldb-aarch64-ubuntu` while building `llvm` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/59/builds/13557

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

```
Step 6 (test) failure: build (failure)
...
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests/5/11 (2087 of 2097)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests/7/11 (2088 of 2097)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests/9/11 (2089 of 2097)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests/8/11 (2090 of 2097)
PASS: lldb-unit :: tools/lldb-server/tests/./LLDBServerTests/0/2 (2091 of 2097)
PASS: lldb-unit :: tools/lldb-server/tests/./LLDBServerTests/1/2 (2092 of 2097)
PASS: lldb-unit :: Target/./TargetTests/11/14 (2093 of 2097)
PASS: lldb-unit :: Host/./HostTests/2/13 (2094 of 2097)
PASS: lldb-unit :: Process/gdb-remote/./ProcessGdbRemoteTests/8/9 (2095 of 2097)
PASS: lldb-api :: tools/lldb-server/TestLldbGdbServer.py (2096 of 2097)
command timed out: 2400 seconds without output running [b'ninja', b'check-lldb'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2707.129204

```

</details>

https://github.com/llvm/llvm-project/pull/129272