[SILOptimizer] Added option to force FSO for uncalled functions.

The new flag -sil-fso-optimize-if-not-called forced function signature
optimization to run even on functions which are not called.  Doing so is
helpful for tests to alleviate the burden of writing code to actually
call a function in whose function signature optimization we are
interested.

Author: nate-chandler

lib/SILOptimizer/FunctionSignatureTransforms/FunctionSignatureOpts.cpp

@@ -70,6 +70,12 @@ static llvm::cl::opt<bool>
                       llvm::cl::desc("Support function signature optimization "
                                      "of generic functions"));
 
+static llvm::cl::opt<bool>
+    FSOOptimizeIfNotCalled("sil-fso-optimize-if-not-called",
+                           llvm::cl::init(false),
+                           llvm::cl::desc("Optimize even if a function isn't "
+                                          "called. For testing only!"));
+
 static bool isSpecializableRepresentation(SILFunctionTypeRepresentation Rep,
                                           bool OptForPartialApply) {
   switch (Rep) {
@@ -632,6 +638,9 @@ bool FunctionSignatureTransform::run(bool hasCaller) {
     return false;
   }
 
+  // If we are asked to assume a caller for testing purposes, set the flag.
+  hasCaller |= FSOOptimizeIfNotCalled;
+
   if (!hasCaller && (F->getDynamicallyReplacedFunction() ||
                      canBeCalledIndirectly(F->getRepresentation()))) {
     LLVM_DEBUG(llvm::dbgs() << "  function has no caller -> abort\n");