fuzzgen: Disable verifier after NaN Canonicalization (#4914)

* fuzzgen: Disable verifier after NaN Canonicalization

We are currently running the verifier twice, once after the nan canonicalization pass, and again when JIT compiling the code.

The verifier first runs in the NaN Canonicalization pass. If it fails it prevents us from getting a nice `cargo fuzz fmt` test case.

So disable the verifier there, but ensure its enabled when JIT compiling.

* fuzzgen: Force enable verifier in cranelift-icache

This is already the default, but since we no longer run the verifier in `fuzzgen` its important to ensure that it runs in the fuzz targets.

fuzz/fuzz_targets/cranelift-icache.rs

@@ -20,6 +20,10 @@ fuzz_target!(|func: SingleFunction| {
         let mut builder = settings::builder();
         // We need llvm ABI extensions for i128 values on x86
         builder.set("enable_llvm_abi_extensions", "true").unwrap();
+
+        // This is the default, but we should ensure that it wasn't accidentally turned off anywhere.
+        builder.set("enable_verifier", "true").unwrap();
+
         builder
     });