optimizer: bail out of inlining if ir_inline_unionsplit will fail

[vtjnash]

Intersection cannot deal with this metharg, so it does not simplify
the type at all when handling this case. This can cause us to run into
an assertion later, where we assume the intersection of a non-Varags
type will always return a simple DataType without Varargs.

Fixes #44238

atype = Tuple{typeof(Base.similar),
  Tuple{Union{Polyhedra.Polyhedron{T}, Polyhedra.Representation{T}} where T},
  Array{_A, 1} where _A,
  Array{_C, 1} where _C,
  Array{_B, 1} where _B}

metharg = Tuple{typeof(Base.similar),
  Tuple{Vararg{Union{Polyhedra.Polyhedron{T}, Polyhedra.Representation{T}} where T}},
  Vararg{Union{Union{AbstractArray{var"#s14", 1}, Polyhedra.AbstractRepIterator{var"#s13", var"#s14"} where var"#s13", Polyhedra.AllRepIterator{var"#s14", var"#s14", LinElemT, LRT, RT} where RT<:Polyhedra.AbstractRepIterator{var"#s14", var"#s14"} where LRT<:Polyhedra.AbstractRepIterator{var"#s14", LinElemT} where LinElemT where var"#s14"} where var"#s14"<:(Polyhedra.HyperPlane{T, AT} where AT<:AbstractArray{T, 1}), Union{AbstractArray{var"#s14", 1}, Polyhedra.AbstractRepIterator{var"#s13", var"#s14"} where var"#s13", Polyhedra.AllRepIterator{var"#s14", var"#s14", LinElemT, LRT, RT} where RT<:Polyhedra.AbstractRepIterator{var"#s14", var"#s14"} where LRT<:Polyhedra.AbstractRepIterator{var"#s14", LinElemT} where LinElemT where var"#s14"} where var"#s14"<:(Polyhedra.HalfSpace{T, AT} where AT<:AbstractArray{T, 1}), Union{AbstractArray{var"#s14", 1}, Polyhedra.AbstractRepIterator{var"#s13", var"#s14"} where var"#s13", Polyhedra.AllRepIterator{var"#s14", var"#s14", LinElemT, LRT, RT} where RT<:Polyhedra.AbstractRepIterator{var"#s14", var"#s14"} where LRT<:Polyhedra.AbstractRepIterator{var"#s14", LinElemT} where LinElemT where var"#s14"} where var"#s14"<:AbstractArray{T, 1}, Union{AbstractArray{var"#s14", 1}, Polyhedra.AbstractRepIterator{var"#s13", var"#s14"} where var"#s13", Polyhedra.AllRepIterator{var"#s14", var"#s14", LinElemT, LRT, RT} where RT<:Polyhedra.AbstractRepIterator{var"#s14", var"#s14"} where LRT<:Polyhedra.AbstractRepIterator{var"#s14", LinElemT} where LinElemT where var"#s14"} where var"#s14"<:(Polyhedra.Line{T, AT} where AT<:AbstractArray{T, 1}), Union{AbstractArray{var"#s14", 1}, Polyhedra.AbstractRepIterator{var"#s13", var"#s14"} where var"#s13", Polyhedra.AllRepIterator{var"#s14", var"#s14", LinElemT, LRT, RT} where RT<:Polyhedra.AbstractRepIterator{var"#s14", var"#s14"} where LRT<:Polyhedra.AbstractRepIterator{var"#s14", LinElemT} where LinElemT where var"#s14"} where var"#s14"<:(Polyhedra.Ray{T, AT} where AT<:AbstractArray{T, 1})} where T}}

Currently typeintersection(atype, metharg) === metharg

[aviatesk]

Even when match.fully_covers, spec_types can contain Varargs?

julia> @eval Core.Compiler let
           foo(a...) = length(a)
           matches = _methods_by_ftype(Tuple{typeof(foo),Int,Vararg{Int}}, -1, get_world_counter())
           matches[1]
       end
       
Core.MethodMatch(Tuple{Core.Compiler.var"#foo#431", Int64, Vararg{Int64}}, svec(), (::Core.Compiler.var"#foo#431")(a...) in Core.Compiler at none:2, true)

[vtjnash]

If it fully covers, then we don't need inline the subtyping computation later (which is what failed)