Pass no-verify-fixpoint option to instcombine in LLVM 18
When LLVM 18 is used, pass the no-verify-fixpoint option when running the instcombine pass. Otherwise LLVM may abort with an error.
The background here is that this option is enabled by default for testing purposes, because instcombine is normally only explicitly invoked like this inside tests. If it is used in an actual production pipeline, the no-verify-fixpoint option needs to be enabled.
This should fix the issue reported at https://bugzilla.redhat.com/show_bug.cgi?id=2268800.