I just tested Debian's 18.1.0-1 package (also built against LLVM 6.0 (package
version 1:6.0-3+b1)) and that version of Mesa doesn't produce the glitch
either, thus narrowing the regression space to:
- Mesa: between 18.1 and current HEAD of Git
- LLVM: between 6.0 and current HEAD of SVN.

Further testing with a version of Debian's 18.1.0-1 package built against LLVM
7 (SVN r333339) didn't exhibit the bug either, therefore the bug must have been
introduced between Mesa 18.1.0 and the current Git HEAD of Mesa. I'll see, if I
can do a bisection.