[MathExtras] Fix UB in minIntN

We negated a value with a signed type which invited problems when that
value was the most negative signed number.  Use an unsigned type
for the value instead.  It will compute the same twos complement
result without the UB.

llvm-svn: 275815

llvm/include/llvm/Support/MathExtras.h

@@ -337,7 +337,7 @@ inline uint64_t maxUIntN(uint64_t N) {
 inline int64_t minIntN(int64_t N) {
   assert(N > 0 && N <= 64 && "integer width out of range");
 
-  return -(INT64_C(1)<<(N-1));
+  return -(UINT64_C(1)<<(N-1));
 }
 
 /// Gets the maximum value for a N-bit signed integer.