LibJS: Generate bytecode in basic blocks instead of one big block

This limits the size of each block (currently set to 1K), and gets us
closer to a canonical, more easily analysable bytecode format.
As a result of this, "Labels" are now simply entries to basic blocks.
Since there is no more 'conditional' jump (as all jumps are always
taken), JumpIf{True,False} are unified to JumpConditional, and
JumpIfNullish is renamed to JumpNullish.
Also fixes #7914 as a result of reimplementing the loop logic.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -1,7 +1,7 @@
 set(SOURCES
         AST.cpp
         Bytecode/ASTCodegen.cpp
-        Bytecode/Block.cpp
+        Bytecode/BasicBlock.cpp
         Bytecode/Generator.cpp
         Bytecode/Instruction.cpp
         Bytecode/Interpreter.cpp