Write isUInt using template specializations to work around an incorrect MSVC warning.

[jlebar]

Summary

Per D22441, MSVC warns on our old implementation of isUInt<64>. It sees
uint64_t(1) << 64 and doesn't realize that it's not going to be
executed. Writing as a template specialization is ugly, but prevents
the warning.

[majnemer]

LGTM

include/llvm/Support/MathExtras.h

 inline bool isShiftedInt(int64_t x) {
   static_assert(
       N > 0, "isShiftedInt<0> doesn't make sense (refers to a 0-bit number.");
   static_assert(N + S <= 64, "isShiftedInt<N, S> with N + S > 64 is too wide.");
   return isInt<N + S>(x) && (x % (UINT64_C(1) << S) == 0);
 }
 
 /// isUInt - Checks if an unsigned integer fits into the given bit width.
-template<unsigned N>
-inline bool isUInt(uint64_t x) {
-  static_assert(N > 0, "isUInt<0> doesn't make sense.");
-  return N >= 64 || x < (UINT64_C(1)<<(N));
+///
+/// This is written as two functions rather than as simply
+///
+///   return N >= 64 || X < (UINT64_C(1) << N);
+///
+/// to keep MSVC from (incorrectly) warning on isUInt<64> that we're shifting
+/// left too many places.
+template <unsigned N>
+inline typename std::enable_if<(N < 64), bool>::type isUInt(uint64_t X) {
+  static_assert(N > 0, "isUInt<0> doesn't make sense");
+  return X < (UINT64_C(1) << N);
+}
+template <unsigned N>
+inline typename std::enable_if<N >= 64, bool>::type isUInt(uint64_t X) {
+  return true;
 }
 
 // Template specializations to get better code for common cases.
 template<>
 inline bool isUInt<8>(uint64_t x) {
   return static_cast<uint8_t>(x) == x;
 }
 template<>