Mandatory Inlining: Use loadFunction() instead of linkFunction()

We only need to deserialize the function itself, not its transitive
dependencies. Also, only deserialize a function after we've checked
that its transparent.

For now, this doesn't reduce the volume of SIL linking, because the
mandatory linker pass still links everything. But we're almost
there.

Author: slavapestov

lib/SILOptimizer/Mandatory/MandatoryInlining.cpp

@@ -404,17 +404,22 @@ static SILFunction *getCalleeFunction(
     return nullptr;
   }
 
-  // If CalleeFunction is a declaration, see if we can load it. If we fail to
-  // load it, bail.
-  if (CalleeFunction->empty()
-      && !AI.getModule().linkFunction(CalleeFunction, Mode))
-    return nullptr;
-
   // If the CalleeFunction is a not-transparent definition, we can not process
   // it.
   if (CalleeFunction->isTransparent() == IsNotTransparent)
     return nullptr;
 
+  // If CalleeFunction is a declaration, see if we can load it.
+  if (CalleeFunction->empty()) {
+    // FIXME: Remove 'Mode'
+    if (Mode != SILOptions::LinkingMode::LinkNone)
+      AI.getModule().loadFunction(CalleeFunction);
+  }
+
+  // If we fail to load it, bail.
+  if (CalleeFunction->empty())
+    return nullptr;
+
   if (F->isSerialized() &&
       !CalleeFunction->hasValidLinkageForFragileInline()) {
     if (!CalleeFunction->hasValidLinkageForFragileRef()) {