LibJS/Bytecode: Store labels as basic block index during compilation

Instead of storing a BasicBlock* and forcing the size of Label to be
sizeof(BasicBlock*), we now store the basic block index as a u32.

This means the final version of the bytecode is able to keep labels
at sizeof(u32), shrinking the size of many instructions. :^)

Userland/Libraries/LibJS/Bytecode/BasicBlock.cpp

@@ -10,13 +10,14 @@
 
 namespace JS::Bytecode {
 
-NonnullOwnPtr<BasicBlock> BasicBlock::create(String name)
+NonnullOwnPtr<BasicBlock> BasicBlock::create(u32 index, String name)
 {
-    return adopt_own(*new BasicBlock(move(name)));
+    return adopt_own(*new BasicBlock(index, move(name)));
 }
 
-BasicBlock::BasicBlock(String name)
-    : m_name(move(name))
+BasicBlock::BasicBlock(u32 index, String name)
+    : m_index(index)
+    , m_name(move(name))
 {
 }