SILOptimizer: Move the StringOptimization a bit earlier in the pipeline.

Needed to make sure that global initializers are not optimized in mid-level SIL while other functions are still in high-level SIL.
Having the StringOptimization not in high-level SIL was just a mistake in my earlier PR.

Author: eeckstein

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -493,6 +493,8 @@ static void addHighLevelFunctionPipeline(SILPassPipelinePlan &P) {
   addFunctionPasses(P, OptimizationLevelKind::HighLevel);
 
   addHighLevelLoopOptPasses(P);
+  
+  P.addStringOptimization();
 }
 
 // After "high-level" function passes have processed the entire call tree, run
@@ -529,8 +531,6 @@ static void addSerializePipeline(SILPassPipelinePlan &P) {
 static void addMidLevelFunctionPipeline(SILPassPipelinePlan &P) {
   P.startPipeline("MidLevel,Function", true /*isFunctionPassPipeline*/);
 
-  P.addStringOptimization();
-
   addFunctionPasses(P, OptimizationLevelKind::MidLevel);
 
   // Specialize partially applied functions with dead arguments as a preparation