LibJS: Add an EliminateLoads pass to Bytecode

This pass tries to eliminate repeated lookups of variables by name, by
remembering where these where last loaded to.

For now the lookup cache needs to be fully cleared with each call or
property access, because we do not have a way to check if these have any
side effects on the currently visible scopes.

Note that property accesses can cause getters/setters to be called, so
these are treated as calls in all cases.

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -233,6 +233,7 @@ Bytecode::PassManager& Interpreter::optimization_pipeline(Interpreter::Optimizat
         pm->add<Passes::MergeBlocks>();
         pm->add<Passes::GenerateCFG>();
         pm->add<Passes::PlaceBlocks>();
+        pm->add<Passes::EliminateLoads>();
     } else {
         VERIFY_NOT_REACHED();
     }