inference: make getfield_tfunc more robust for abstract PartialStruct (…

…JuliaLang#34541)

We're currently careful never to make these. But good to be careful?

refs JuliaLang#34513

base/compiler/tfuncs.jl

@@ -826,17 +826,18 @@ function getfield_tfunc(@nospecialize(s00), @nospecialize(name))
  end
  end
  s = typeof(sv)
- elseif isa(s, PartialStruct)
+ elseif isa(s00, PartialStruct)
+ s = widenconst(s00)
+ sty = unwrap_unionall(s)::DataType
  if isa(name, Const)
  nv = name.val
  if isa(nv, Symbol)
- nv = fieldindex(widenconst(s), nv, false)
+ nv = fieldindex(sty, nv, false)
  end
- if isa(nv, Int) && 1 <= nv <= length(s.fields)
- return unwrapva(s.fields[nv])
+ if isa(nv, Int) && 1 <= nv <= length(s00.fields)
+ return unwrapva(s00.fields[nv])
  end
  end
- s = widenconst(s)
  end
  if isType(s) || !isa(s, DataType) || isabstracttype(s)
  return Any