add some nospecialization hacks

[aviatesk]

The previous @nospecialize(::Type) dispatch signatures actually disabled
nospecializations, and we see following kind of dynamic dispatches:

julia> using JET

julia> report_opt((Vector{Any},)) do ts
           isbitstype(ts[1])
       end
═════ 1 possible error found ═════
┌ @ REPL[3]:2 Main.isbitstype(%1)
│ runtime dispatch detected: Main.isbitstype(%1::Any)
└─────────────

This commit also adds an overload of emptymutable(::IdSet), which
eliminates the dynamic dispatch below:

julia> report_opt((Base.IdSet{Any}, Base.IdSet{Any}); annotate_types=true) do s1, s2
           s1 ∪ s2
       end
═════ 2 possible errors found ═════
┌ @ REPL[3]:2 Main.∪(s1::Base.IdSet{Any}, s2::Base.IdSet{Any})
│┌ @ abstractset.jl:48 Base.union!(Core.tuple(Base.emptymutable(s::Base.IdSet{Any}, Base.promote_eltype(Core.tuple(s::Base.IdSet{Any})::Tuple{Base.IdSet{Any}}, sets::Tuple{Base.IdSet{Any}}...)::Type{Any})::Set{Any}, s::B>
││┌ @ abstractset.jl:76 Base.union!(s::Set{Any}, x::Base.IdSet{Any})
│││┌ @ abstractset.jl:93 Base.sizehint!(s::Set{Any}, Base.+(Base.length(s::Set{Any})::Int64, Core.typeassert(Base.Int(Base.length(itr::Base.IdSet{Any})::Int64)::Int64, Base.Int)::Int64)::Int64)
││││┌ @ set.jl:84 Base.sizehint!(Base.getproperty(s::Set{Any}, :dict::Symbol)::Dict{Any, Nothing}, newsz::Int64)
│││││┌ @ dict.jl:244 Base.rehash!(d::Dict{Any, Nothing}, newsz::Int64)
││││││┌ @ dict.jl:203 Base.hashindex(%231::Any, %20::Int64)
│││││││ runtime dispatch detected: Base.hashindex(%231::Any, %20::Int64)
││││││└───────────────
│││┌ @ abstractset.jl:95 Base.push!(s::Set{Any}, %62::Any)
││││ runtime dispatch detected: Base.push!(s::Set{Any}, %62::Any)
│││└─────────────────────

[vtjnash]

It doesn't cause any sort of specialization, though it did inhibit inlining:

julia> mt = typeof(isbitstype).name.mt;

julia> mt.leafcache
Any[]

julia> mt.cache
Core.TypeMapEntry(nothing, Tuple{typeof(isbitstype), Type}, Tuple{typeof(isbitstype), Any}, svec(), 0x00000000000012d4, 0xffffffffffffffff, MethodInstance for isbitstype(::Type), false, false, false)

[aviatesk]

So does that mean the runtime avoids specialization when a dispatch signature contains Type as an heuristic?
I thought a dynamic dispatch usually causes a specialization.