LibJS/Bytecode: Don't replace the entry block in MergeBlocks

The entry block must stay in place, although it's okay to merge stuff
into it.

This fixes 4 test262 tests and brings us to parity with optimization
disabled. :^)

Author: awesomekling

Userland/Libraries/LibJS/Bytecode/Pass/MergeBlocks.cpp

@@ -36,7 +36,8 @@ void MergeBlocks::perform(PassPipelineExecutable& executable)
         if (entry.key->terminator()->type() != Instruction::Type::Jump)
             continue;
 
-        {
+        // NOTE: We can't replace the first block in a function, as it's the entry block.
+        if (entry.key != executable.executable.basic_blocks.first()) {
             InstructionStreamIterator it { entry.key->instruction_stream() };
             auto& first_instruction = *it;
             if (first_instruction.type() == Instruction::Type::Jump) {