Disable check_label_branch_invariants in fuzzing

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.

cranelift/codegen/src/machinst/buffer.rs

@@ -334,7 +334,7 @@ impl<I: VCodeInst> MachBuffer<I> {
     /// times, e.g. after calling `add_{cond,uncond}_branch()` and
     /// before emitting branch bytes.
     fn check_label_branch_invariants(&self) {
-        if !cfg!(debug_assertions) {
+        if !cfg!(debug_assertions) || cfg!(fuzzing) {
             return;
         }
         let cur_off = self.cur_offset();