Fix leaking of Nops

We would leak Nops into FuzzIL programs in the
MinimizationPostProcessor that were not removed.
This would lead to Nops being marked as .notRemovable in subsequent
minimization attempts which would break some assertions when trying
to remove Nops as we never expect them to have the .notRemovable
flag set.

Sources/Fuzzilli/Minimization/MinimizationHelper.swift

@@ -42,6 +42,7 @@ class MinimizationHelper {
     private static let defaultNumExecutions = 1
 
     init(for aspects: ProgramAspects, forCode code: Code, of fuzzer: Fuzzer, runningOnFuzzerQueue: Bool) {
+        assert(code.filter({ $0.isNop }).count == 0)
         self.aspects = aspects
         self.fuzzer = fuzzer
         self.runningOnFuzzerQueue = runningOnFuzzerQueue

Sources/Fuzzilli/Minimization/MinimizationPostProcessor.swift

@@ -100,9 +100,7 @@ struct MinimizationPostProcessor {
             // Either we're adding a new instruction (in which case we're replacing a nop inserted in step 1), or changing the number of inputs of an existing instruction.
             assert((helper.code[change.index].op is Nop && !(change.newInstruction.op is Nop)) ||
                    (helper.code[change.index].op.name == change.newInstruction.op.name && helper.code[change.index].numInputs < change.newInstruction.numInputs))
-            if !helper.tryReplacing(instructionAt: change.index, with: change.newInstruction) {
-                return false
-            }
+            helper.tryReplacing(instructionAt: change.index, with: change.newInstruction)
         }
 
         // Step 3: Remove any remaining nops from step 1.

Sources/Fuzzilli/Minimization/Minimizer.swift

@@ -112,6 +112,7 @@ public class Minimizer: ComponentBase {
         postProcessor.process(with: helper)
 
         assert(helper.code.isStaticallyValid())
+        assert(!helper.code.contains(where: { $0.isNop }))
 
         return helper.finalize()
     }