[llvm] [SelectionDAG]Salvage debuginfo when combining load and sext instrs. (PR #169779)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 166417 tests passed
* 2869 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### LLVM
<details>
<summary>LLVM.DebugInfo/X86/selectionDAG-load-sext.mir</summary>

```
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 2
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/llc -O2 /Users/srastogi/Development/Delta/test.mir -mtriple=x86_64-unkown-linux -start-before=x86-isel -stop-after=x86-isel -o - | /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/DebugInfo/X86/selectionDAG-load-sext.mir --check-prefix=MIR
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/llc -O2 /Users/srastogi/Development/Delta/test.mir -mtriple=x86_64-unkown-linux -start-before=x86-isel -stop-after=x86-isel -o -
# .---command stderr------------
# | /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/llc: error: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/llc: /Users/srastogi/Development/Delta/test.mir: error: Could not open input file: No such file or directory
# `-----------------------------
# error: command failed with exit status: 1
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/DebugInfo/X86/selectionDAG-load-sext.mir --check-prefix=MIR
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/FileCheck /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/test/DebugInfo/X86/selectionDAG-load-sext.mir --check-prefix=MIR
# `-----------------------------
# error: command failed with exit status: 2

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/169779