Fix TODO in IncrementalCompact's already_inserted (JuliaLang#49380)

The `already_inserted` query on `IncrementalCompact` had a missing
case for being called on incremental compact whose underlying IR
had new nodes to be inserted. This does not happen in the base pipeline,
because `already_inserted` is only used in the sroa passes, but can
happen in some non-Base pipelines that run multiple rounds of sroa.

Author: Keno

base/compiler/ssair/passes.jl

@@ -326,7 +326,7 @@ function already_inserted(compact::IncrementalCompact, old::OldSSAValue)
     end
     id -= length(compact.ir.stmts)
     if id < length(compact.ir.new_nodes)
-        error("")
+        return already_inserted(compact, OldSSAValue(compact.ir.new_nodes.info[id].pos))
     end
     id -= length(compact.ir.new_nodes)
     @assert id <= length(compact.pending_nodes)