[mandatory-combine] Make sure to bail early if we have an external declaration.

Beyond creating a nice invariant, I am going to be using this invariant in some
small code improvements to MandatoryCombine that assume we have an initial basic
block.

Author: gottesmm

lib/SILOptimizer/Mandatory/MandatoryCombine.cpp

@@ -282,6 +282,12 @@ class MandatoryCombine final : public SILFunctionTransform {
   void run() override {
     auto *function = getFunction();
 
+    // If this function is an external declaration, bail. We only want to visit
+    // functions with bodies.
+    if (function->isExternalDeclaration()) {
+      return;
+    }
+
     MandatoryCombiner combiner(createdInstructions);
     bool madeChange = combiner.runOnFunction(*function);