Small refactoring of make_mlir_fn + more

[jumerckx]

This pr splits the implementation of make_mlir_fn into multiple parts:

1. prepare_mlir_fn_args
2. process_linear_args
3. trace the function by calling it
4. finalize_mlir_fn

This is a prerequisite for doing automatic function call insertion as 1., 2., and 4. can be repurposed there. I'd like to merge this already in order to prevent constant merge conflicts as I make progress on the call insertions.

Unrelated, I moved code in Ops.call around, and did slight refactoring in utils.jl by introducing push_inst!, both also useful for the call insertion. If any of these changes turn out to be more contentious than the others, I can split this pr as well.

[jumerckx]

I'm not sure if CI failures are related to this pr?

[wsmoses]

DUS: Error During Test at /home/runner/work/Reactant.jl/Reactant.jl/test/optimize_comm.jl:64
  Got exception outside of a @test
  UndefVarError: `ctx` not defined
  Stacktrace:
    [1] finalize_mlir_fn(result::Nothing, traced_args::Vector{Any}, linear_args::Vector{Union{ReactantCore.MissingTracedValue, Reactant.TracedRArray, Reactant.TracedRNumber}}, seen_args::Reactant.OrderedIdDict{Any, Any}, seen_results::Reactant.OrderedIdDict{Any, Any}, fnbody::Reactant.MLIR.IR.Block, func::Reactant.MLIR.IR.Operation, mod::Reactant.MLIR.IR.Module, name::String, in_tys::Vector{Reactant.MLIR.IR.Type}, do_transpose::Bool, optimize_then_pad::Bool, inv_map::IdDict{Any, Any}, args_in_result::Symbol, resprefix::Symbol, argprefix::Symbol, resargprefix::Symbol, verify_arg_names::Nothing, return_dialect::Symbol, traced_args_to_shardings::Reactant.OrderedIdDict{Any, Any}, output_shardings::Nothing, sym_visibility::Nothing, num_replicas::Int64, runtime::Val{:PJRT}, construct_function_without_args::Bool, args::Tuple{ConcretePJRTArray{Int64, 2, 8, Reactant.Sharding.ShardInfo{Reactant.Sharding.NamedSharding{2, Reactant.Sharding.Mesh{2, Vector{Int64}}}, Vector{Tuple{UnitRange{Int64}, UnitRange{Int64}}}}}, ConcretePJRTArray{Int64, 2, 8, Reactant.Sharding.ShardInfo{Reactant.Sharding.NamedSharding{2, Reactant.Sharding.Mesh{2, Vector{Int64}}}, Vector{Tuple{UnitRange{Int64}, UnitRange{Int64}}}}}}, N::Int64, concretein::Bool, toscalar::Bool)
      @ Reactant.TracedUtils ~/work/Reactant.jl/Reactant.jl/src/TracedUtils.jl:801
    [2] make_mlir_fn(f::typeof(Main.var"##Comm Optimization#243".dus), args::Tuple{ConcretePJRTArray{Int64, 2, 8, Reactant.Sharding.ShardInfo{Reactant.Sharding.NamedSharding{2, Reactant.Sharding.Mesh{2, Vector{Int64}}}, Vector{Tuple{UnitRange{Int64}, UnitRange{Int64}}}}}, ConcretePJRTArray{Int64, 2, 8, Reactant.Sharding.ShardInfo{Reactant.Sharding.NamedSharding{2, Reactant.Sharding.Mesh{2, Vector{Int64}}}, Vector{Tuple{UnitRange{Int64}, UnitRange{Int64}}}}}}, kwargs::@NamedTuple{}, name::String, concretein::Bool; toscalar::Bool, return_dialect::Symbol, args_in_result::Symbol, construct_function_without_args::Bool, do_transpose::Bool, input_shardings::Nothing, output_shardings::Nothing, runtime::Val{:PJRT}, verify_arg_names::Nothing, argprefix::Symbol, resprefix::Symbol, resargprefix::Symbol, num_replicas::Int64, optimize_then_pad::Bool)
      @ Reactant.TracedUtils ~/work/Reactant.jl/Reactant.jl/src/TracedUtils.jl:332
      ```
      
      seems related?