inference: enable constant propagation for union-split signatures

The inference precision of certain functions really relies on constant
propagation, but currently constant prop' won't happen when a call
signature is union split and so sometimes inference ends up looser
return type: e.g.
```julia
julia>
Base.return_types((Union{Tuple{Int,Nothing},Tuple{Int,Missing}},)) do t
           a, b = t
           a # I expected a::Int, but a::Union{Missing,Nothing,Int}
       end |> first Union{Missing, Nothing, Int64}
```

This PR:
- enables constant prop' for each union signatures, by calling
  `abstract_call_method_with_const_args` just after each
  `abstract_call_method`
- refactor `abstract_call_method_with_const_args` into two separate
  parts, 1.) heuristics to decide whether to do constant prop', 2.) try
  constant propagation

The added test cases will should showcase the cases where the
inference result could be improved by that.

---

I've not seen notable regression in latency with this PR.
Here is a sample benchmark of the impact of this PR on latency,
from which I guess this PR is acceptable ?

> build time: master (caeacef)
```bash
Sysimage built. Summary:
Total ───────  61.615938 seconds
Base: ───────  26.575732 seconds 43.1313%
Stdlibs: ────  35.038024 seconds 56.8652%
    JULIA usr/lib/julia/sys-o.a
Generating REPL precompile statements... 30/30
Executing precompile statements... 1378/1378
Precompilation complete. Summary:
Total ─────── 116.417013 seconds
Generation ──  81.077365 seconds 69.6439%
Execution ───  35.339648 seconds 30.3561%
    LINK usr/lib/julia/sys.dylib
```

> build time: this PR
```bash
Stdlibs total  ──── 34.077962 seconds
Sysimage built. Summary:
Total ───────  61.804573 seconds
Base: ───────  27.724077 seconds 44.8576%
Stdlibs: ────  34.077962 seconds 55.1383%
    JULIA usr/lib/julia/sys-o.a
Generating REPL precompile statements... 30/30
Executing precompile statements... 1362/1362
Precompilation complete. Summary:
Total ─────── 111.262672 seconds
Generation ──  83.535305 seconds 75.0794%
Execution ───  27.727367 seconds 24.9206%
    LINK usr/lib/julia/sys.dylib
```

> first time to plot: master (caeacef)
```julia
julia> using Plots; @time plot(rand(10,3))
  3.614168 seconds (5.47 M allocations: 324.564 MiB, 5.73% gc time,
53.02% compilation time)
```

> first time to plot: this PR
```julia
julia> using Plots; @time plot(rand(10,3))
  3.557919 seconds (5.53 M allocations: 328.812 MiB, 2.89% gc time,
51.94% compilation time)
```

---

- fixes #37610
- some part of this code was taken from #37637
- this PR is originally supposed to be alternative and more generalized
  version of #39296