[llvm] [SelectionDAG] Salvage debug info for non-constant ADDs (PR #68981)

[David Stenberg via llvm-commits]

dstenb wrote:

The test case was generated using `-stop-before=sparc-isel`, which produced MIR output with no body for the function. Running `-start-before=sparc-isel` on that output (as was done in the test case) resulted in the following verifier error which broke the expensive-checks bots:

```
*** Bad machine code: Function has NoVRegs property but there are VReg operands ***
- function:    pointer_add_unknown_offset
LLVM ERROR: Found 1 machine code errors.
```

I have extracted the IR code from the test file, and changed that a .ll test instead, still starting before sparc-isel. I saw that there are other tests starting before isel that looks like that, e.g. `llvm/test/CodeGen/AArch64/callbr-asm-outputs-indirect-isel.ll`. I verified that this works with `update_mir_test_checks.py` (the only difference is that the CHECKs are inserted in the IR code instead).

The patch was re-landed with that change in 22f1217932e11672a337e0a5504cd5b2eb3fedf8.

https://github.com/llvm/llvm-project/pull/68981