[llvm] [SystemZ] Don't crash on undef source in shouldCoalesce() (PR #78056)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

> @JonPsson1 The test you added is leaving a .s file around which is causing an unresolved test error on a buildbot because the .s file has no RUN lines:
> 
> https://lab.llvm.org/buildbot/#/builders/275/builds/3332
> 
> ```
> ******************** TEST 'LLVM :: CodeGen/SystemZ/regcoal_undefsrc.s' FAILED ********************
> Test has no 'RUN:' line
> ********************
> ```

Thanks - fixed with e2ce91f.

https://github.com/llvm/llvm-project/pull/78056