Add tests for max/minIntN(64).

[jlebar]

Summary

Given that we had a bug on max/minUIntN(64), these should have tests
too.

llvm/trunk/unittests/Support/MathExtrasTest.cpp

TEST(MathExtras, isUIntN) {
   EXPECT_FALSE(isUIntN(16, 65536));
   EXPECT_TRUE(isUIntN(1, 0));
   EXPECT_TRUE(isUIntN(6, 63));
 }
 
 TEST(MathExtras, maxIntN) {
   EXPECT_EQ(32767, maxIntN(16));
   EXPECT_EQ(2147483647, maxIntN(32));
+  EXPECT_EQ(std::numeric_limits<int32_t>::max(), maxIntN(32));
+  EXPECT_EQ(std::numeric_limits<int64_t>::max(), maxIntN(64));
 }
 
 TEST(MathExtras, minIntN) {
   EXPECT_EQ(-32768LL, minIntN(16));
   EXPECT_EQ(-64LL, minIntN(7));
+  EXPECT_EQ(std::numeric_limits<int32_t>::min(), minIntN(32));
+  EXPECT_EQ(std::numeric_limits<int64_t>::min(), minIntN(64));
 }
 
 TEST(MathExtras, maxUIntN) {
   EXPECT_EQ(0xffffULL, maxUIntN(16));
   EXPECT_EQ(0xffffffffULL, maxUIntN(32));
   EXPECT_EQ(0xffffffffffffffffULL, maxUIntN(64));
   EXPECT_EQ(1ULL, maxUIntN(1));
   EXPECT_EQ(0x0fULL, maxUIntN(4));