Run LLVM verify pass on IR in assert builds.

Run LLVM verify pass on IR in assert builds.

The problem fixed by commit 53c8d6c9 would have been noticed if we'd
been running LLVM's verify pass on generated IR.  Doing so also reveals
a complaint about incorrect name mangling, fixed here.  Only enabled for
LLVM 17+ because it uses the new pass manager API.

Suggested-by: Dmitry Dolgov <9erthalion6@gmail.com>
Discussion: https://postgr.es/m/CAFj8pRACpVFr7LMdVYENUkScG5FCYMZDDdSGNU-tch%2Bw98OxYg%40mail.gmail.com