widenconst of a Const(::Type) can be quite slow

[Keno]

Currently a lot of our code is written with the assumption that widenconst(::Const) is extremely fast. However, this is not true if the value inside the Const happens to be a Type. There, it can be very slow:

c1 = Core.Const(Base.RefValue)
c2 = Core.Const(2)

julia> @btime Core.Compiler.widenconst(c1)
  4.540 μs (0 allocations: 0 bytes)
Type{Base.RefValue}

julia> @btime Core.Compiler.widenconst(c2)
  40.162 ns (0 allocations: 0 bytes)
Int64

[vtjnash]

Refs #42596?

[Keno]

Perhaps, but I think even a single type cache lookup can be quite problematic for these. I'm playing with just remembering the pointer in the types itself.

[Keno]

In fact, as it stands, this issue might make #42596 problematic, because it might force computation of the widenconst value even when it isn't otherwise required.

[vtjnash]

I think that change might see widenconst return the kind instead, with the type-level information consistently tracked elsewhere instead