[SIL] ApplySite: Get args by callee index.

Added convenience functions to ApplySite to access argument and argument
operand by index into the callee's argument list (rather than by index
into the arguments used by the apply instruction).

Author: nate-chandler

include/swift/SIL/ApplySite.h

@@ -295,9 +295,20 @@ class ApplySite {
   /// Return the apply operand for the given applied argument index.
   Operand &getArgumentRef(unsigned i) const { return getArgumentOperands()[i]; }
 
+  // The apply operand at the given index into the callee's function's
+  // arguments.
+  Operand &getArgumentRefAtCalleeArgIndex(unsigned i) const {
+    return getArgumentRef(i - getCalleeArgIndexOfFirstAppliedArg());
+  }
+
   /// Return the ith applied argument.
   SILValue getArgument(unsigned i) const { return getArguments()[i]; }
 
+  // The argument at the given index into the callee's function's arguments.
+  SILValue getArgumentAtCalleeArgIndex(unsigned i) const {
+    return getArgument(i - getCalleeArgIndexOfFirstAppliedArg());
+  }
+
   /// Set the ith applied argument.
   void setArgument(unsigned i, SILValue V) const {
     getArgumentOperands()[i].set(V);