refactor(levm): remove unneeded checks in swap (#5305)

**Motivation**

Currently for swapN we do two unnecessary checks:
- `stack.len() < N`, which is also done inside `Stack::swap`
- checked add of offset (stack depth) and N, which is guaranteed to
never fail as N and the offset are both bounded

**Description**

Checking that `offset + N >= STACK_DEPTH` should be enough.

Author: iovoid

crates/vm/levm/src/call_frame.rs

@@ -144,11 +144,15 @@ impl Stack {
     }
 
     #[inline(always)]
-    pub fn swap(&mut self, index: usize) -> Result<(), ExceptionalHalt> {
-        let index = self
-            .offset
-            .checked_add(index)
-            .ok_or(ExceptionalHalt::StackUnderflow)?;
+    pub fn swap<const N: usize>(&mut self) -> Result<(), ExceptionalHalt> {
+        // Compile-time check that ensures `self.offset + N` is safe,
+        // since self.offset is bounded by STACK_LIMIT
+        const {
+            assert!(STACK_LIMIT.checked_add(N).is_some());
+        }
+        #[expect(clippy::arithmetic_side_effects)]
+        let index = self.offset + N;
+
         if index >= self.values.len() {
             return Err(ExceptionalHalt::StackUnderflow);
         }

crates/vm/levm/src/opcode_handlers/exchange.rs

@@ -1,5 +1,5 @@
 use crate::{
-    errors::{ExceptionalHalt, OpcodeResult, VMError},
+    errors::{OpcodeResult, VMError},
     gas_cost,
     vm::VM,
 };
@@ -13,10 +13,7 @@ impl<'a> VM<'a> {
         let current_call_frame = &mut self.current_call_frame;
         current_call_frame.increase_consumed_gas(gas_cost::SWAPN)?;
 
-        if current_call_frame.stack.len() < N {
-            return Err(ExceptionalHalt::StackUnderflow.into());
-        }
-        current_call_frame.stack.swap(N)?;
+        current_call_frame.stack.swap::<N>()?;
 
         Ok(OpcodeResult::Continue)
     }