[LazyCallGraph] Update libcall list when replacing a libcall node's function

[aeubanks]

Summary

Otherwise when we visit all libcalls in
updateCGAndAnalysisManagerForPass(), the old libcall is dead and doesn't
have a node.

We treat libcalls conservatively in LazyCallGraph because any function
may introduce calls to them out of thin air.

It is weird to change the signature of a libcall since introducing calls
to the libcall with a different signature may break, but other passes
like deadargelim already do it, so let's preserve this behavior for now.

Fixes an issue found in D128830.

[psamolysov]

@aeubanks Thank you very much for the fix! I tried the cases described in D128830 and to run the new test Analysis/LazyCallGraph/replace-libcall.ll. On my environment, everything works fine, so I accept the revision.

llvm/lib/Analysis/LazyCallGraph.cpp

assert(OldF.use_empty() &&
          "Must have moved all uses from the old function to the new!");
 #endif
 
   N.replaceFunction(NewF);
 
   // Update various call graph maps.
   G->NodeMap.erase(&OldF);
   G->NodeMap[&NewF] = &N;
+
+  // Update lib functions.
+  if (G->isLibFunction(OldF)) {
+    G->LibFunctions.remove(&OldF);
+    G->LibFunctions.insert(&NewF);
+  }
 }
 
 void LazyCallGraph::insertEdge(Node &SourceN, Node &TargetN, Edge::Kind EK) {
   assert(SCCMap.empty() &&
          "This method cannot be called after SCCs have been formed!");
 
   return SourceN->insertEdgeInternal(TargetN, EK);
 }

llvm/test/Analysis/LazyCallGraph/replace-libcall.ll

+; RUN: opt -passes=inline,argpromotion < %s -S | FileCheck %s
+
+; Make sure we update the list of libcalls when we replace a libcall.
+
+; CHECK: define {{.*}}@a
+
+define void @a() {
+entry:
+  %call = call float @strtof(ptr noundef null, ptr noundef null)
+  ret void
+}
+
+define internal float @strtof(ptr noundef %0, ptr noundef %1) nounwind {
+entry:
+  ret float 0.0
+}
+