[sil-func-extractor] Rather than manually creating a pass manager just to run DFE, just

add a high level API to perform the operation.

Author: gottesmm

include/swift/SILOptimizer/PassManager/Passes.h

@@ -44,6 +44,9 @@ namespace swift {
   /// user code.
   void performSILDiagnoseUnreachable(SILModule *M);
 
+  /// \brief Remove dead functions from \p M.
+  void performSILDeadFunctionElimination(SILModule *M);
+
   /// \brief Link a SILFunction declaration to the actual definition in the
   /// serialized modules.
   ///

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -604,3 +604,9 @@ SILTransform *swift::createDeadFunctionElimination() {
 SILTransform *swift::createExternalFunctionDefinitionsElimination() {
   return new SILExternalFuncDefinitionsElimination();
 }
+
+void swift::performSILDeadFunctionElimination(SILModule *M) {
+  SILPassManager PM(M, "SIL Dead Function Elimination");
+  PM.addDeadFunctionElimination();
+  PM.runOneIteration();
+}

tools/sil-func-extractor/SILFunctionExtractor.cpp

@@ -215,9 +215,7 @@ void removeUnwantedFunctions(SILModule *M, ArrayRef<std::string> MangledNames,
   }
 
   // Remove dead functions.
-  SILPassManager PM(M);
-  PM.addDeadFunctionElimination();
-  PM.run();
+  performSILDeadFunctionElimination(M);
 }
 
 int main(int argc, char **argv) {