This commit improves SROA pass by extending the `unswitchtupleunion`
optimization to handle the general parametric types, e.g.:
```julia
julia> struct A{T}
           x::T
       end;

julia> function foo(a1, a2, c)
           t = c ? A(a1) : A(a2)
           return getfield(t, :x)
       end;

julia> only(Base.code_ircode(foo, (Int,Float64,Bool); optimize_until="SROA"))
```

> Before
```
2 1 ─      goto #3 if not _4                                          │
  2 ─ %2 = %new(A{Int64}, _2)::A{Int64}                               │╻ A
  └──      goto #4                                                    │
  3 ─ %4 = %new(A{Float64}, _3)::A{Float64}                           │╻ A
  4 ┄ %5 = φ (#2 => %2, #3 => %4)::Union{A{Float64}, A{Int64}}        │
3 │   %6 = Main.getfield(%5, :x)::Union{Float64, Int64}               │
  └──      return %6                                                  │
   => Union{Float64, Int64}
```

> After
```
julia> only(Base.code_ircode(foo, (Int,Float64,Bool); optimize_until="SROA"))
2 1 ─      goto #3 if not _4                                           │
  2 ─      nothing::A{Int64}                                           │╻ A
  └──      goto #4                                                     │
  3 ─      nothing::A{Float64}                                         │╻ A
  4 ┄ %8 = φ (#2 => _2, #3 => _3)::Union{Float64, Int64}               │
  │        nothing::Union{A{Float64}, A{Int64}}
3 │   %6 = %8::Union{Float64, Int64}                                   │
  └──      return %6                                                   │
   => Union{Float64, Int64}
```