isolate: keep CSVs used by helpers

We used not to put CSVs in `declarations-only`, since `lift` is able to
create them. However, this led to have less CSVs than those we get
post-inlining (some are only used in helpers). This represents a problem
since sometimes we ignore non-existing registers.

lib/FunctionIsolation/IsolateFunctions.cpp

@@ -781,6 +781,19 @@ private:
             CalledFunctions.insert(CalledFunction);
         }
 
+        // Preserve all the CSVs used by helpers as well, this saves us from
+        // having "surprises" later, which can be a problem since sometimes we
+        // ignore non-existing CSVs.
+        if (auto *Call = getCallToHelper(&Instruction)) {
+          auto MaybeUsedCSVs = tryGetCSVUsedByHelperCall(Call);
+          if (MaybeUsedCSVs) {
+            for (auto *CSV : MaybeUsedCSVs->Read)
+              UsedGVs.insert(CSV);
+            for (auto *CSV : MaybeUsedCSVs->Written)
+              UsedGVs.insert(CSV);
+          }
+        }
+
         std::queue<llvm::User *> Users;
         Users.push(&Instruction);
 

lib/HelperArgumentsAnalysis/HelpersModuleToDeclarations.cpp

@@ -29,11 +29,14 @@ public:
     }
 
     // The global variables are not needed in this module. Drop their debug info
-    // and their initializer. GlobalDCE will be run later to remove them. This
-    // is except a couple of special ones that are always needed.
+    // and their initializer. GlobalDCE will be run later to remove them.
+    // Note that we preserve CSVs and some other special variables. In
+    // particular, we need CSVs to avoid having new CSVs popping up
+    // post-inlining, which is problematic since sometimes we handle
+    // non-existing CSVs in a special way.
     for (GlobalVariable &GV : M.globals()) {
       GV.eraseMetadata(LLVMContext::MD_dbg);
-      if (not isSpecialGV(GV))
+      if (not isSpecialGV(GV) and not FunctionTags::CSV.isTagOf(&GV))
         GV.setInitializer(nullptr);
     }