MathExtras UnitTest: Assert that isPowerOf2(0) is false. NFC.

[zvi]

Summary

This is a follow-up on D34077. Elena observed that the
correctness of the code relies on isPowerOf2(0) returning false.
Adding a test to cover this corner-case.

[davide]

LGTM as this is actually documenting the current behaviour more explicity.
Feel free to commit cleanup like this without a pre-commit review (somebody will complain post commit if needed :)

[zvi]

Thanks :)

llvm/trunk/unittests/Support/MathExtrasTest.cpp

TEST(MathExtras, reverseBits) {
   uint64_t NZ64 = 42;
   EXPECT_EQ(0x54ULL, reverseBits(NZ8));
   EXPECT_EQ(0x5400ULL, reverseBits(NZ16));
   EXPECT_EQ(0x54000000ULL, reverseBits(NZ32));
   EXPECT_EQ(0x5400000000000000ULL, reverseBits(NZ64));
 }
 
 TEST(MathExtras, isPowerOf2_32) {
+  EXPECT_FALSE(isPowerOf2_32(0));
   EXPECT_TRUE(isPowerOf2_32(1 << 6));
   EXPECT_TRUE(isPowerOf2_32(1 << 12));
   EXPECT_FALSE(isPowerOf2_32((1 << 19) + 3));
   EXPECT_FALSE(isPowerOf2_32(0xABCDEF0));
 }
 
 TEST(MathExtras, isPowerOf2_64) {
+  EXPECT_FALSE(isPowerOf2_64(0));
   EXPECT_TRUE(isPowerOf2_64(1LL << 46));
   EXPECT_TRUE(isPowerOf2_64(1LL << 12));
   EXPECT_FALSE(isPowerOf2_64((1LL << 53) + 3));
   EXPECT_FALSE(isPowerOf2_64(0xABCDEF0ABCDEF0LL));
 }
 
 TEST(MathExtras, PowerOf2Ceil) {
   EXPECT_EQ(0U, PowerOf2Ceil(0U));