Add precompiled tags if backedge is missing (#15)

In JuliaLang/julia#49617 it was found that
not all callees have backedges to callers. There is some speculation
that rather than adding the backedge to those cases, we may in fact
be able to prune the backedge graph more substantially in cases where
it may be safe to avoid propagating invalidation.

Consequently, to enforce the guarantees made by `@compile_workload`,
PrecompileTools should check the inference graph and add tags in cases
where a backedge is missing.

src/PrecompileTools.jl

@@ -17,12 +17,31 @@ function workload_enabled(mod::Module)
     end
 end
 
+"""
+    check_edges(node)
+
+Recursively ensure that all callees of `node` are precompiled. This is (rarely) necessary
+because sometimes there is no backedge from callee to caller (xref https://github.com/JuliaLang/julia/issues/49617),
+and `staticdata.c` relies on the backedge to trace back to a MethodInstance that is tagged `mi.precompiled`.
+"""
+function check_edges(node)
+    parentmi = node.mi_info.mi
+    for child in node.children
+        childmi = child.mi_info.mi
+        if !(isdefined(childmi, :backedges) && parentmi ∈ childmi.backedges)
+            precompile(childmi.specTypes)
+        end
+        check_edges(child)
+    end
+end
+
 function precompile_roots(roots)
     @assert have_inference_tracking
     for child in roots
         mi = child.mi_info.mi
         precompile(mi.specTypes) # TODO: Julia should allow one to pass `mi` directly (would handle `invoke` properly)
         verbose[] && println(mi)
+        check_edges(child)
     end
 end