feat[venom]: reduce legacy opts when venom is enabled

reduce legacy IR optimization when venom is enabled. the use of
`IRnode._optimized` was there to decide if it was safe to inline an
IRnode expression or it required a `with` statement; with venom, we
don't need to inline the expressions, since the venom optimizer is more
powerful. this leads to a 50% improvement in AST -> IRnode generation,
which is a ~15% performance improvement in overall end-to-end compile
time.

vyper/codegen/ir_node.py

@@ -6,7 +6,7 @@
 from typing import Any, List, Optional, Union
 
 import vyper.ast as vy_ast
-from vyper.compiler.settings import VYPER_COLOR_OUTPUT
+from vyper.compiler.settings import VYPER_COLOR_OUTPUT, get_global_settings
 from vyper.evm.address_space import AddrSpace
 from vyper.evm.opcodes import get_ir_opcodes
 from vyper.exceptions import CodegenPanic, CompilerPanic
@@ -426,6 +426,10 @@ def is_pointer(self) -> bool:
 
     @property  # probably could be cached_property but be paranoid
     def _optimized(self):
+        if get_global_settings().experimental_codegen:
+            # in venom pipeline, we don't need to inline constants.
+            return self
+
         # TODO figure out how to fix this circular import
         from vyper.ir.optimizer import optimize