IR: Set a TargetPrefix for nvvm intrinsics

Since these are named nvvm_* rather than nvptx_*, we also need to
update getArchTypePrefix. It's a bit unusual for getArchTypePrefix not
to match the backend name, but I think this fits the intent of the
function in this case.

llvm-svn: 274890
diff --git a/llvm/lib/Support/Triple.cpp b/llvm/lib/Support/Triple.cpp
index 433d146..cfa12a9 100644
--- a/llvm/lib/Support/Triple.cpp
+++ b/llvm/lib/Support/Triple.cpp
@@ -114,8 +114,9 @@
 
   case xcore:       return "xcore";
 
-  case nvptx:       return "nvptx";
-  case nvptx64:     return "nvptx";
+  // NVPTX intrinsics are namespaced under nvvm.
+  case nvptx:       return "nvvm";
+  case nvptx64:     return "nvvm";
 
   case le32:        return "le32";
   case le64:        return "le64";