effects: improve :consistent-cy analysis on getfield

[aviatesk]

This commit improves the accuracy of the :consistent-cy effect analysis
by handling getfield accessing local mutable objects.

The existing analysis taints :consistent-cy upon any getfield call
accessing mutable object because we really don't have a knowledge about
the object lifetime and so we need to conservatively take into account a
possibility of the mutable object being a global variable.

However we can "recover" :consistent-cy tainted by access to mutable
object when the newly added :noglobal helper effect has been proven
because in that case we can conclude that all mutable objects accessed
within the method are purely local and thus :consistent (more
precisely we also need to confirm that all the call arguments are known
not to be mutable global objects to derive this conclusion).

For example now we can prove :consistent-cy of the function below and
it will be concrete-evaluated:

julia> @noinline function mutable_consistent(s)
           broadcast(identity, Ref(s))
       end
mutable_consistent (generic function with 1 method)

julia> Base.infer_effects(mutable_consistent, (String,))
(+c,+e,!n,+t,+s,+g)

julia> code_typed() do
           mutable_consistent(:foo)
       end
1-element Vector{Any}:
 CodeInfo(
1 ─     return :foo
) => Symbol

[vtjnash]

Suggested change

	ty === Module && return true # TODO add comment on why this is valid
	ty === Module && return true # there is no inferable constructor (with effect_free consistency), so this object must have already existed