inference: don't forget to add backedge for effect-free frame

[aviatesk]

Now our abstract interpretation system caches two separate lattice
properties return type and call effects, and the previous optimization
to avoid adding backedge for a frame whose return type is Any turns
out to be insufficient -- now we also need to make sure that the
inferred effects don't provide any useful IPO information.

Example:

julia> const CONST_DICT = let d = Dict()
           for c in 'A':'z'
               push!(d, c => Int(c))
           end
           d
       end;

julia> Base.@assume_effects :total_may_throw getcharid(c) = CONST_DICT[c];

julia> @noinline callf(f, args...) = f(args...);

julia> function entry_to_be_invalidated(c)
           return callf(getcharid, c)
       end;

julia> @test Base.infer_effects((Char,)) do x
           entry_to_be_invalidated(x)
       end |> Core.Compiler.is_concrete_eval_eligible
Test Passed

julia> @test fully_eliminated(; retval=97) do
           entry_to_be_invalidated('a')
       end
Test Passed

julia> getcharid(c) = CONST_DICT[c]; # now this is not eligible for concrete evaluation

julia> @test Base.infer_effects((Char,)) do x
           entry_to_be_invalidated(x)
       end |> !Core.Compiler.is_concrete_eval_eligible
Test Failed at REPL[10]:1
  Expression: Base.infer_effects((Char,)) do x
        entry_to_be_invalidated(x)
    end |> !(Core.Compiler.is_concrete_eval_eligible)
ERROR: There was an error during testing

julia> @test !fully_eliminated() do
           entry_to_be_invalidated('a')
       end
Test Failed at REPL[11]:1
  Expression: !(fully_eliminated() do
        entry_to_be_invalidated('a')
    end)
ERROR: There was an error during testing