subtype: replace leaf-bound typevars if they would result in Tuple{Union{}} otherwise

[vtjnash]

This was a primary motivation for #49111. Previously, we'd see some some methods like convert(::Type{T}, ::T) where T<:Float64 (apparently from tuple convert specializations), which were not necessary

julia> which(Tuple{Type{T}, T} where {T<:Number}).specializations
svec(MethodInstance for Float64(::Float64), MethodInstance for AbstractFloat(::Float64), MethodInstance for AbstractFloat(::BigFloat), MethodInstance for AbstractFloat(::Float32), MethodInstance for Float32(::Float32), MethodInstance for Float16(::Float16), MethodInstance for Int64(::Int64), nothing, nothing, nothing)

[vtjnash]

Note that the existing Core.Compiler.normalize_typevars fails this case because it has:

julia> (Tuple{Type{T},T} where T<:Int) <: Tuple{Type{Int},Int}
false

[vtjnash]

@nanosoldier runbenchmarks(!"scalar", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.

[N5N3]

@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.