inference: use ssa_def_slot for typeassert refinement

[aviatesk]

Allows type refinement in the following kind of case:

julia> @test Base.infer_return_type((Vector{Any},)) do args
           codeinst = first(args)
           if codeinst isa Core.MethodInstance
               mi = codeinst
           else
               codeinst::Core.CodeInstance
               mi = codeinst.def
           end
           return mi
       end == Core.MethodInstance
Test Passed