JuliaLang / julia / #37074

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optimizer: SROA `mutable(immutable(...))` case correctly (#43239)

Our SROA should be able to handle `mutable(immutable(...))` case even
without "proper" alias analysis, since we eliminate `immutable` first
and then process `mutable` within our iterative approach.

It turns out it didn't work because after the immutable handling
we keep the reference count _before_ DCE, but didn't update dead 
reference count. This commit fixes it and now we should be able to 
eliminate more allocations, e.g.:
```julia
julia> simple_sroa(s) = broadcast(identity, Ref(s))
simple_sroa (generic function with 1 method)

julia> simple_sroa("compile"); @allocated(simple_sroa("julia"))
0
```

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