Disable function signature optimization at -Osize.

We might be more aggressive here in the future.

lib/SILOptimizer/Transforms/FunctionSignatureOpts.cpp

@@ -1140,6 +1140,11 @@ public:
   void run() override {
     auto *F = getFunction();
 
+    // Don't run function signature optimizations at -Os.
+    if (F->getModule().getOptions().Optimization ==
+        SILOptions::SILOptMode::OptimizeForSize)
+      return;
+
     // Don't optimize callees that should not be optimized.
     if (!F->shouldOptimize())
       return;