Fix another wasm-mutate invalid module (#1797)

Return a first-class error instead of generating an invalid module
during loop unrolling in wasm-mutate when unsupported opcodes are
encountered.

Author: alexcrichton

crates/wasm-mutate/src/mutators/codemotion/loop_unrolling.rs

@@ -18,7 +18,7 @@ use crate::{
         },
         OperatorAndByteOffset,
     },
-    WasmMutate,
+    Error, WasmMutate,
 };
 
 /// This mutator selects a random `loop` construction in a function and tries to unroll it.
@@ -110,6 +110,14 @@ impl LoopUnrollWriter {
 
                             to_fix.insert(idx, Instruction::BrTable(jmpfix.into(), def));
                         }
+
+                        Operator::BrOnCast { .. }
+                        | Operator::BrOnCastFail { .. }
+                        | Operator::BrOnNull { .. }
+                        | Operator::BrOnNonNull { .. } => {
+                            log::info!("unsupported operator {op:?}");
+                            return Err(Error::no_mutations_applicable());
+                        }
                         _ => {}
                     }
                 }