Use a faster implementation of maxUIntN.

Summary:
On x86-64 with clang 3.8, before:

   mov     edx, 1
   mov     cl, dil
   shl     rdx, cl
   cmp     rdi, 64
   mov     rax, -1
   cmovne  rax, rdx
   ret

after:

  mov     ecx, 64
  sub     ecx, edi
  mov     rax, -1
  shr     rax, cl
  ret

Reviewers: rnk

Subscribers: dylanmckay, mkuper, llvm-commits

Differential Revision: https://reviews.llvm.org/D22440

llvm-svn: 275718