Do not memcpy with same source and destination.

This can happen for sret_args. Drop the assignment in this case.

gen/tollvm.cpp

@@ -545,6 +545,8 @@ void DtoMemSetZero(LLValue* dst, LLValue* nbytes)
 
 void DtoMemCpy(LLValue* dst, LLValue* src, LLValue* nbytes, unsigned align)
 {
+    assert (src != dst && "src and dst of memcpy must be different");
+
     LLType* VoidPtrTy = getVoidPtrType();
 
     dst = DtoBitCast(dst, VoidPtrTy);