This commit allows elimination of mutable φ-node (and its predecessor mutables allocations).
As an contrived example, it allows this `mutable_ϕ_elim(::String, ::Vector{String})`
to run without any allocations at all:
```julia
function mutable_ϕ_elim(x, xs)
    r = Ref(x)
    for x in xs
        r = Ref(x)
    end
    return r[]
end

let xs = String[string(gensym()) for _ in 1:100]
    mutable_ϕ_elim("init", xs)
    @test @allocated(mutable_ϕ_elim("init", xs)) == 0
end
```

This mutable ϕ-node elimination is still limited though.
Most notably, the current implementation doesn't work if a mutable
allocation forms multiple ϕ-nodes, since we check allocation eliminability
(i.e. escapability) by counting usages counts and thus it's hard to
reason about multiple ϕ-nodes at a time.
For example, currently mutable allocations involved in cases like below
will still not be eliminated:
```julia
code_typed((Bool,String,String),) do cond, x, y
    if cond
        ϕ2 = ϕ1 = Ref(x)
    else
        ϕ2 = ϕ1 = Ref(y)
    end
    ϕ1[], ϕ2[]
end

\# more realistic example
mutable struct Point{T}
    x::T
    y::T
end
add(a::Point, b::Point) = Point(a.x + b.x, a.y + b.y)
function compute(a::Point{ComplexF64}, b::Point{ComplexF64})
    for i in 0:(100000000-1)
        a = add(add(a, b), b)
    end
    a.x, a.y
end
```

I'd say this limitation should be addressed by first introducing a better
abstraction for reasoning escape information. More specifically, I'd like
introduce EscapeAnalysis.jl into Julia base first, and then gradually
adapt it to improve our SROA pass, since EA will allow us to reason about
all escape information imposed on whatever object more easily and should
help us get rid of the complexities of our current SROA implementation.

For now, I'd like to get in this enhancement even though it has the
limitation elaborated above, as far as this commit doesn't introduce
latency problem (which is unlikely).