simplify call to promote_eltype with repeated elements (#51135)

Helps to short-circuit calls to large splat calls, since those have all
the same type elements.

Fixes #51011

(cherry picked from commit 3527213)

base/abstractarray.jl

@@ -1591,10 +1591,14 @@ eltypeof(x::AbstractArray) = eltype(x)
 promote_eltypeof() = error()
 promote_eltypeof(v1) = eltypeof(v1)
 promote_eltypeof(v1, vs...) = promote_type(eltypeof(v1), promote_eltypeof(vs...))
+promote_eltypeof(v1::T, vs::T...) where {T} = eltypeof(v1)
+promote_eltypeof(v1::AbstractArray{T}, vs::AbstractArray{T}...) where {T} = T
 
 promote_eltype() = error()
 promote_eltype(v1) = eltype(v1)
 promote_eltype(v1, vs...) = promote_type(eltype(v1), promote_eltype(vs...))
+promote_eltype(v1::T, vs::T...) where {T} = eltype(T)
+promote_eltype(v1::AbstractArray{T}, vs::AbstractArray{T}...) where {T} = T
 
 #TODO: ERROR CHECK
 _cat(catdim::Int) = Vector{Any}()