[MSan] Don't create zero offsets in getShadowPtrForArgument(). NFC

[glider]

Summary

For MSan instrumentation with MS.ParamTLS and MS.ParamOriginTLS being TLS variables, the CreateAdd() with ArgOffset==0 is a no-op, because the compiler is able to fold the addition of 0.
But for KMSAN, which receives ParamTLS and ParamOriginTLS from a call to the runtime library, this introduces a stray instruction which complicates reading/testing the IR.

[glider]

Formatted the patch with clang-format

[glider]

Committed revision 327829.

lib/Transforms/Instrumentation/MemorySanitizer.cpp

void insertWarningFn(IRBuilder<> &IRB, Value *Origin) {
   }
 
   /// \brief Compute the shadow address for a given function argument.
   ///
   /// Shadow = ParamTLS+ArgOffset.
   Value *getShadowPtrForArgument(Value *A, IRBuilder<> &IRB,
                                  int ArgOffset) {
     Value *Base = IRB.CreatePointerCast(MS.ParamTLS, MS.IntptrTy);
+    if (ArgOffset)
         Base = IRB.CreateAdd(Base, ConstantInt::get(MS.IntptrTy, ArgOffset));
     return IRB.CreateIntToPtr(Base, PointerType::get(getShadowTy(A), 0),
                               "_msarg");
   }
 
   /// \brief Compute the origin address for a given function argument.
   Value *getOriginPtrForArgument(Value *A, IRBuilder<> &IRB,
                                  int ArgOffset) {
     if (!MS.TrackOrigins) return nullptr;
     Value *Base = IRB.CreatePointerCast(MS.ParamOriginTLS, MS.IntptrTy);
+    if (ArgOffset)
         Base = IRB.CreateAdd(Base, ConstantInt::get(MS.IntptrTy, ArgOffset));
     return IRB.CreateIntToPtr(Base, PointerType::get(MS.OriginTy, 0),
                               "_msarg_o");
   }
 
   /// \brief Compute the shadow address for a retval.
   Value *getShadowPtrForRetval(Value *A, IRBuilder<> &IRB) {
     return IRB.CreatePointerCast(MS.RetvalTLS,
                                  PointerType::get(getShadowTy(A), 0),