optimizer: some cosmetic improvements on inlining code (#43115)
- remove an unnecessary field and allocation
- make the signature order consistent across the file:
`ir::IRCode, idx::Int, stmt, rt, info, flag::UInt8, sig::Signature, state::InliningState, todo::Vector{Pair{Int, Any}}`
- pass the same `stmt::Expr` object and slightly improve its type-stability
