optimizer: enable SROA with constant globals (JuliaLang#42355)

```julia
julia> const REF_FLD = :x;

julia> code_typed() do
           r = Ref{Int}(42) # should be eliminated
           x = getfield(r, REF_FLD) # should be eliminated
           return sin(x)
       end |> only
CodeInfo(
1 ─ %1 = Base.sitofp(Float64, 42)::Float64
│   %2 = invoke Base.Math.sin(%1::Float64)::Float64
└──      return %2
) => Float64
```

base/compiler/ssair/passes.jl

@@ -19,11 +19,27 @@ struct SSADefUse
 end
 SSADefUse() = SSADefUse(Int[], Int[], Int[])
 
-try_compute_fieldidx_expr(typ::DataType, expr::Expr) = try_compute_fieldidx_args(typ, expr.args)
-function try_compute_fieldidx_args(typ::DataType, args::Vector{Any})
-    field = args[3]
-    isa(field, QuoteNode) && (field = field.value)
+function try_compute_field_stmt(compact::IncrementalCompact, stmt::Expr)
+    field = stmt.args[3]
+    # fields are usually literals, handle them manually
+    if isa(field, QuoteNode)
+        field = field.value
+    elseif isa(field, Int)
+    # try to resolve other constants, e.g. global reference
+    else
+        field = compact_exprtype(compact, field)
+        if isa(field, Const)
+            field = field.val
+        else
+            return nothing
+        end
+    end
     isa(field, Union{Int, Symbol}) || return nothing
+    return field
+end
+
+function try_compute_fieldidx_stmt(compact::IncrementalCompact, stmt::Expr, typ::DataType)
+    field = try_compute_field_stmt(compact, stmt)
     return try_compute_fieldidx(typ, field)
 end
 
@@ -636,10 +652,8 @@ function getfield_elim_pass!(ir::IRCode)
         else
             continue
         end
-        ## Normalize the field argument to getfield/setfield
-        field = stmt.args[3]
-        isa(field, QuoteNode) && (field = field.value)
-        isa(field, Union{Int, Symbol}) || continue
+        field = try_compute_field_stmt(compact, stmt)
+        field === nothing && continue
 
         struct_typ = unwrap_unionall(widenconst(compact_exprtype(compact, stmt.args[2])))
         if isa(struct_typ, Union) && struct_typ <: Tuple
@@ -779,13 +793,13 @@ function getfield_elim_pass!(ir::IRCode)
             # it would have been deleted. That's fine, just ignore
             # the use in that case.
             stmt === nothing && continue
-            field = try_compute_fieldidx_expr(typ, stmt)
+            field = try_compute_fieldidx_stmt(compact, stmt::Expr, typ)
             field === nothing && (ok = false; break)
             push!(fielddefuse[field].uses, use)
         end
         ok || continue
         for use in defuse.defs
-            field = try_compute_fieldidx_expr(typ, ir[SSAValue(use)])
+            field = try_compute_fieldidx_stmt(compact, ir[SSAValue(use)]::Expr, typ)
             field === nothing && (ok = false; break)
             push!(fielddefuse[field].defs, use)
         end

test/compiler/irpasses.jl

@@ -383,3 +383,45 @@ exc39508 = ErrorException("expected")
     return err
 end
 @test test39508() === exc39508
+
+let # `getfield_elim_pass!` should work with constant globals
+    # immutable pass
+    src = @eval Module() begin
+        const REF_FLD = :x
+        struct ImmutableRef{T}
+            x::T
+        end
+
+        code_typed((Int,)) do x
+            r = ImmutableRef{Int}(x) # should be eliminated
+            x = getfield(r, REF_FLD) # should be eliminated
+            return sin(x)
+        end |> only |> first
+    end
+    @test !any(src.code) do @nospecialize(stmt)
+        Meta.isexpr(stmt, :call) || return false
+        ft = Core.Compiler.argextype(stmt.args[1], src, Any[], src.slottypes)
+        return Core.Compiler.widenconst(ft) == typeof(getfield)
+    end
+    @test !any(src.code) do @nospecialize(stmt)
+        return Meta.isexpr(stmt, :new)
+    end
+
+    # mutable pass
+    src = @eval Module() begin
+        const REF_FLD = :x
+        code_typed() do
+            r = Ref{Int}(42) # should be eliminated
+            x = getfield(r, REF_FLD) # should be eliminated
+            return sin(x)
+        end |> only |> first
+    end
+    @test !any(src.code) do @nospecialize(stmt)
+        Meta.isexpr(stmt, :call) || return false
+        ft = Core.Compiler.argextype(stmt.args[1], src, Any[], src.slottypes)
+        return Core.Compiler.widenconst(ft) == typeof(getfield)
+    end
+    @test !any(src.code) do @nospecialize(stmt)
+        return Meta.isexpr(stmt, :new)
+    end
+end