Don't clean up LLVM state when exiting in a bad way

Hi,

I ran into some segfaults when using Postgres that was compiled with LLVM 7. According to the backtraces these crashes happened during the call to llvm_shutdown, during cleanup after another out of memory condition. It seems that calls to LLVMOrcDisposeInstance, can crash (at least on LLVM 7) when LLVM is left in bad state. I attached the relevant part of the stacktrace to this email.

With the attached patch these segfaults went away. The patch turns llvm_shutdown into a no-op whenever the backend is exiting with an error. Based on my understanding of the code this should be totally fine. No memory should be leaked, since all memory will be cleaned up anyway once the backend exits shortly after. The only reason this cleanup code even seems to exist at all is to get useful LLVM profiling data. To me it seems be acceptable if the profiling data is incorrect/missing when the backend exits with an error.

Jelte