LibJS: Add LoadArgument bytecode instruction for fast argument access

This is generated for Identifier nodes that represent a function
argument variable. It loads a given argument index from the current
call frame into the accumulator.

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -61,6 +61,7 @@
     O(Decrement)                  \
     O(Throw)                      \
     O(PushLexicalEnvironment)     \
+    O(LoadArgument)               \
     O(EnterUnwindContext)         \
     O(LeaveUnwindContext)         \
     O(ContinuePendingUnwind)      \