[AMDGPU] Give enum an explicit 64-bit type to fix MSVC 2013 failures

Recall that MSVC always gives enums the type 'int', nothing else.  MSVC
2015 does not appear to have this problem anymore.

Clang-cl -Wmicrosoft-enum-value flags this, FWIW, so now I have a true
positive for my warning. :)

llvm-svn: 278762

llvm/lib/Target/AMDGPU/R600InstrInfo.h

@@ -21,7 +21,7 @@
 namespace llvm {
 
 namespace R600InstrFlags {
-enum {
+enum : uint64_t {
  REGISTER_STORE = UINT64_C(1) << 62,
  REGISTER_LOAD = UINT64_C(1) << 63
 };