irinterp: set IR_FLAG_REFINED for narrowed PhiNodes (#56391)

`adce_pass!` can transform a `Union`-type `PhiNode` into a narrower
`PhiNode`, but in such cases, the `IR_FLAG_REFINED` flag isn’t set on
that `PhiNode` statement. By setting this flag, irinterp can perform
statement reprocessing using the narrowed `PhiNode`, enabling type
stability in cases like #56387.

- fixes #56387

(cherry picked from commit 671e1d8)

Author: aviatesk

base/compiler/ssair/passes.jl

@@ -2075,18 +2075,19 @@ function adce_pass!(ir::IRCode, inlining::Union{Nothing,InliningState}=nothing)
         unionphi = unionphis[i]
         phi = unionphi[1]
         t = unionphi[2]
+        inst = compact.result[phi]
         if t === Union{}
-            stmt = compact[SSAValue(phi)][:stmt]::PhiNode
+            stmt = inst[:stmt]::PhiNode
             kill_phi!(compact, phi_uses, 1:length(stmt.values), SSAValue(phi), stmt, true)
             made_changes = true
             continue
         elseif t === Any
             continue
-        elseif ⊑(𝕃ₒ, compact.result[phi][:type], t)
-            continue
         end
+        ⊏ = strictpartialorder(𝕃ₒ)
+        t ⊏ inst[:type] || continue
         to_drop = Int[]
-        stmt = compact[SSAValue(phi)][:stmt]
+        stmt = inst[:stmt]
         stmt === nothing && continue
         stmt = stmt::PhiNode
         for i = 1:length(stmt.values)
@@ -2098,7 +2099,8 @@ function adce_pass!(ir::IRCode, inlining::Union{Nothing,InliningState}=nothing)
                 push!(to_drop, i)
             end
         end
-        compact.result[phi][:type] = t
+        inst[:type] = t
+        add_flag!(inst, IR_FLAG_REFINED) # t ⊏ inst[:type]
         kill_phi!(compact, phi_uses, to_drop, SSAValue(phi), stmt, false)
         made_changes = true
     end

test/compiler/inference.jl

@@ -5693,3 +5693,15 @@ end === Union{}
 @test Base.infer_return_type() do
     TypeVar(:Issue56248, Any, 1)
 end === Union{}
+
+function issue56387(nt::NamedTuple, field::Symbol=:a)
+    NT = typeof(nt)
+    names = fieldnames(NT)
+    types = fieldtypes(NT)
+    index = findfirst(==(field), names)
+    if index === nothing
+        throw(ArgumentError("Field $field not found"))
+    end
+    types[index]
+end
+@test Base.infer_return_type(issue56387, (typeof((;a=1)),)) == Type{Int}