[function-signature-opts] Commit initial function sig optimization pass that eliminates dead arguments.

The way this pass works is very similar to generic specialization except
that it turns the old function into a thunk that calls the newly created
function that has had the dead arguments removed.

This ensures that any place in the code where we were unable to see that
the old function was being called still works and also enables us to
cut down on code size increase due to code duplication since the
marshalling code for the thunk should be very small.

This is just the first part of a larger body of work that optimizes
function signatures. The plan is to include transforming loadable
pointer args to pass by value and to convert @owned arguments to
@gauranteed arguments.

<rdar://problem/17319928>

Swift SVN r18970

tools/sil-opt/SILOpt.cpp

@@ -70,7 +70,8 @@ enum class PassKind {
   GlobalARCOpts,
   DCE,
   EnumSimplification,
-  LoopInfoPrinter
+  LoopInfoPrinter,
+  FunctionSignatureOpts,
 };
 
 enum class OptGroup {
@@ -229,6 +230,9 @@ Passes(llvm::cl::desc("Passes:"),
                         clEnumValN(PassKind::LoopInfoPrinter,
                                    "loop-info-printer",
                                    "Display loop information."),
+                        clEnumValN(PassKind::FunctionSignatureOpts,
+                                   "function-signature-opts",
+                                   "Optimize function signatures."),
                         clEnumValEnd));
 
 static llvm::cl::opt<bool>
@@ -402,6 +406,9 @@ static void runCommandLineSelectedPasses(SILModule *Module,
     case PassKind::LoopInfoPrinter:
       PM.add(createLoopInfoPrinter());
       break;
+    case PassKind::FunctionSignatureOpts:
+      PM.add(createFunctionSignatureOpts());
+      break;
     }
   }
   PM.run();