[func-sig-opts] Use MutableArrayRef instead of llvm::SmallVector to store Argument/Result descriptors.

We never push_back additional such items onto the descriptor lists once the
transform is constructed. We do on the other hand, potentially modify those
items in the vector. That suggests that this property is better represented by
the "simpler" MutableArrayRef.

Author: gottesmm

lib/SILOptimizer/Transforms/FunctionSignatureOpts.cpp

@@ -113,11 +113,11 @@ class FunctionSignatureTransform {
 
   /// Keep a "view" of precompiled information on arguments that we use 
   /// during our optimization.
-  llvm::SmallVector<ArgumentDescriptor, 4> &ArgumentDescList;
+  MutableArrayRef<ArgumentDescriptor> ArgumentDescList;
 
   /// Keep a "view" of precompiled information on the direct results that we
   /// will use during our optimization.
-  llvm::SmallVector<ResultDescriptor, 4> &ResultDescList;
+  MutableArrayRef<ResultDescriptor> ResultDescList;
 
   /// Return a function name based on ArgumentDescList and ResultDescList.
   std::string createOptimizedSILFunctionName();