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

[via llvm-commits]

dyung 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
********************
```

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