Don't do uint64_t(1) << 64 in maxUIntN.

Summary:
This shift is undefined behavior (and, as compiled by clang, gives the
wrong answer for maxUIntN(64)).

Reviewers: mkuper

Subscribers: llvm-commits, jroelofs, rsmith

Differential Revision: https://reviews.llvm.org/D22430

llvm-svn: 275656
diff --git a/llvm/unittests/Support/MathExtrasTest.cpp b/llvm/unittests/Support/MathExtrasTest.cpp
index 04e1662..ef46311 100644
--- a/llvm/unittests/Support/MathExtrasTest.cpp
+++ b/llvm/unittests/Support/MathExtrasTest.cpp
@@ -131,6 +131,7 @@
 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));
 }