Check whether all fields of a mutable struct are defined before SROA

For now, just bail out in this situation. There's a number of better
things we could do here. However, I want to avoid making JuliaLang#26764 worse
for now.

Fixes JuliaLang#27365

base/compiler/ssair/passes.jl

@@ -696,6 +696,18 @@ function getfield_elim_pass!(ir::IRCode, domtree)
  push!(fielddefuse[field].defs, use)
  end
  ok || continue
+ # Check that the defexpr has defined values for all the fields
+ # we're accessing. In the future, we may want to relax this,
+ # but we should come up with semantics for well defined semantics
+ # for uninitialized fields first.
+ for (fidx, du) in pairs(fielddefuse)
+ isempty(du.uses) && continue
+ if fidx + 1 > length(defexpr.args)
+ ok = false
+ break
+ end
+ end
+ ok || continue
  preserve_uses = IdDict{Int, Vector{Any}}((idx=>Any[] for idx in IdSet{Int}(defuse.ccall_preserve_uses)))
  # Everything accounted for. Go field by field and perform idf
  for (fidx, du) in pairs(fielddefuse)

test/core.jl

@@ -6170,3 +6170,16 @@ translate27368(name::Symbol) =
 translate27368(::Type{Val{name}}) where {name} =
  field27368(name)
 @test isa(translate27368(:name), Combinator27368)
+
+# issue #27365
+mutable struct foo27365
+ x::Float64
+ foo27365() = new()
+end
+
+function baz27365()
+ data = foo27365()
+ return data.x
+end
+
+@test isa(baz27365(), Float64)