Add a simple pass to copy Exprs that appears multiple times in the AST

These objects make it really hard to mutate the AST correctly
since one mutation can be accidentally done at places where it is invalid.

Author: yuyichao

base/inference.jl

@@ -3390,6 +3390,7 @@ function optimize(me::InferenceState)
         # and elide unnecessary allocations
         alloc_elim_pass!(me)
         getfield_elim_pass!(me)
+        copy_duplicated_expr_pass!(me)
         # Clean up for `alloc_elim_pass!` and `getfield_elim_pass!`
         void_use_elim_pass!(me)
         filter!(x -> x !== nothing, code)
@@ -6046,6 +6047,36 @@ function replace_getfield!(e::Expr, tupname, vals, field_names, sv::InferenceSta
     end
 end
 
+function copy_expr_in_array!(ary, seen)
+    for i in 1:length(ary)
+        ex = ary[i]
+        isa(ex, Expr) || continue
+        ex = ex::Expr
+        if haskey(seen, ex)
+            newex = Expr(ex.head)
+            append!(newex.args, ex.args)
+            newex.typ = ex.typ
+            ary[i] = ex = newex
+            # No need to add to `seen`, there's no way there can be another one of the copied
+            # version in the AST....
+        else
+            seen[ex] = nothing
+            if haskey(seen, ex.args)
+                # Haven't actually seen this happen but it's pretty easy to check
+                ex.args = copy(ex.args)
+            else
+                seen[ex.args] = nothing
+            end
+        end
+        copy_expr_in_array!(ex.args, seen)
+    end
+end
+
+# Clone expressions that appears multiple times in the code
+function copy_duplicated_expr_pass!(sv::InferenceState)
+    copy_expr_in_array!(sv.src.code, ObjectIdDict())
+end
+
 # fix label numbers to always equal the statement index of the label
 function reindex_labels!(sv::InferenceState)
     body = sv.src.code