Spurious type instability warning in code_warntype

[yuyichao]

Probably caused by #15609

See below, notice the ::Any after the assignment to _3 and _4. They looks harmless but annoy...

julia> f3(a) = for i in eachindex(a)
           a[i] += 0
       end
f3 (generic function with 1 method)

julia> @code_warntype f3(Float32[])
Variables:
  #self#::#f3
  a::Array{Float32,1}
  #temp#::Int64
  i::Int64

Body:
  begin  # none, line 1: # abstractarray.jl, line 404: # abstractarray.jl, line 414:
      GenSym(3) = (Base.arraylen)(_2::Array{Float32,1})::Int64 # range.jl, line 83:
      GenSym(4) = (Base.select_value)((Base.sle_int)(1,GenSym(3))::Bool,GenSym(3),(Base.box)(Int64,(Base.sub_int)(1,1)))::Int64
      _3::Any = 1
      8: 
      unless (Base.box)(Base.Bool,(Base.not_int)((_3::Int64 === (Base.box)(Int64,(Base.add_int)(GenSym(4),1)))::Bool)) goto 23
      GenSym(5) = _3::Int64
      GenSym(6) = (Base.box)(Int64,(Base.add_int)(_3::Int64,1))
      _4::Any = GenSym(5)
      _3::Any = GenSym(6) # none, line 2: # promotion.jl, line 172: # promotion.jl, line 135:
      Float32
      Float32
      GenSym(2) = (Base.box)(Base.Float32,(Base.add_float)((Base.arrayref)(_2::Array{Float32,1},_4::Int64)::Float32,(Base.box)(Float32,(Base.sitofp)(Float32,0))))
      (Base.arrayset)(_2::Array{Float32,1},GenSym(2),_4::Int64)::Array{Float32,1}
      21: 
      goto 8
      23: 
      24: 
      return
  end::Void