This commit disables the `MachBuffer::check_label_branch_invariants`
debug check on the fuzzers due to it causing timeouts with the test case
from #3441. Fuzzing leads to a 20-30x slowdown of executed code and
locally the fuzz time it takes to instantiate #3441 drops from 3 minutes
to 6 seconds disabling this function. Note that this should still be
executed during our testing on CI since it's still enabled for debug
assertions.