LLVM 18 and later, instcombine perfoms only one iteration.
it performs extra "verify fixpoint" operation when instcombine
is specified in certain ways, including how we do so here.
a problem is that the verification raises a fatal error when it
finds we didn't reach a fixpoint:
LLVM ERROR: Instruction Combining did not reach a fixpoint
after 1 iterations
while it should be rare, it's quite normal not to reach a fixpoint.
this commit fixes the issue by simply disabing the verification.
cf. 41895843b5
c2d7fa30df