yul: Compile objects that carry deploy code only

[dimartiro]

Description

Closes #490

resolc --yul rejected any Yul object that carries deploy code only, that is one without a _deployed runtime sub-object, while solc compiles it:

object "Test" {
    code {
        { mstore(0, 42) return(0, 32) }
    }
}

Error: The contract `deploy_only.yul:Test` unoptimized LLVM IR verification error:
Basic Block in function '__runtime' does not have terminator!
label %entry

Cause

Object::declare declares PolkaVMRuntimeCodeFunction unconditionally, so __runtime always exists with an empty entry block. Object::into_llvm only ever defines it from the _deployed branch, which is reached through if let Some(object) = self.inner_object. With no sub-object the function stays declared and undefined, and the module fails verification.

Not declaring it is not an option: Entry::leave_entry looks __runtime up and bails with Contract runtime code not found, since __entry dispatches to either __deploy or __runtime on the call flag.

Fix

Object::parse now materializes the runtime sub-object when the source omits it, so the AST that reaches code generation always has one.

Doing it in the parser rather than special casing the absence in code generation keeps every consumer on the path it already handles. That matters here because both IR pipelines read this field: newyork's YulTranslator::translate_object has the same if let Some(inner_object) shape, and --newyork reproduces the identical error. One parser-level change fixes both.

The synthesized body is an explicit stop(). Code::into_llvm terminates every code block with an implicit stop anyway ("The EVM lets the code return implicitly"), so this is exactly what an empty runtime block lowers to, and it matches the EVM behaviour of calling an account with no code: success with empty return data.

I did not use an empty block, for two reasons. It would leave __runtime as entry: br %return / return: unreachable, and that unreachable is now reachable rather than a dead tail, which is UB the optimizer is free to exploit — __runtime is Private and called only from __entry, so folding it away could turn a call into a constructor run. And separately, an empty runtime block currently trips an assertion in polkavm-linker under --newyork (see below), which the stop() avoids.

Tests added:

• revive-yul: the deploy-only object gets the implicit runtime object with a stop() body, and an explicit _deployed object is not clobbered by it.
• resolc: compiles_object_without_a_runtime_sub_object compiles the new deploy_only_object.yul fixture end to end and cross-checks that solc agrees on the exit code, which is the premise of the issue.