JIT breaks PostGIS

Today I spent some time closing PostGIS tickets in preparation to Monday's release. 

One of the blockers, https://trac.osgeo.org/postgis/ticket/4125, was filed by Postgres APT repository maintainer Christoph Berg who noticed a test suite failure on Debian Stretch with Postgres 11.

Upon investigation we found:

 - A build for Ubuntu Bionic installed on Debian Stretch passes the suite, requiring llvm6;

 - A build for Debian Stretch fails the suite on a call to external library GEOS, showing no traces of JIT in the stacktrace;

 - Setting jit=off lets the suite pass;

 - The query called in clean session by itself does not crash Postgres. Queries above it are required to reproduce the crash;

 - The crash affects not only Stretch, but customly collected Postgres 11 / clang 3.9 on Travis CI running Ubuntu Trusty: https://github.com/postgis/postgis/pull/262.

I suspect that a fix would require to bisect llvm/clang version which stops showing this behavior and making it a new minimum for JIT, if this is not a symptom of bigger (memory management?) problem.

Thank you!