LibJS: Add a note about inaccuracies to UnifySameBlocks

Author: https://github.com/Hendiadyoin1
Commit: c9e7d452c5
Pull-request: https://github.com/SerenityOS/serenity/pull/15857
Issue: https://github.com/SerenityOS/serenity/issues/15753
Reviewed-by: https://github.com/alimpfard

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

@@ -29,6 +29,8 @@ void UnifySameBlocks::perform(PassPipelineExecutable& executable)
                 continue;
 
             auto candidate_bytes = candidate_block->instruction_stream();
+            // FIXME: NewBigInt's value is not correctly reflected by its encoding in memory,
+            //        this will yield false negatives for blocks containing that
             if (memcmp(candidate_bytes.data(), block_bytes.data(), candidate_block->size()) == 0)
                 equal_blocks.set(&*candidate_block, &block);
         }