LibJS: Optimize reading known-to-be-initialized var bindings

`var` bindings are never in the temporal dead zone (TDZ), and so we
know accessing them will not throw.

We now take advantage of this by having a specialized environment
binding value getter that doesn't check for exceptional cases.

1.08x speedup on JetStream.

Libraries/LibJS/Bytecode/Instruction.h

@@ -69,6 +69,7 @@
     O(GetObjectPropertyIterator)       \
     O(GetPrivateById)                  \
     O(GetBinding)                      \
+    O(GetInitializedBinding)           \
     O(GreaterThan)                     \
     O(GreaterThanEquals)               \
     O(HasPrivateId)                    \