Summary:
This is an optimization to the CTZ primops introduced for #9340

Previously we called out to `hs_ctz64`, but we can actually generate
better hand-tuned code while avoiding the FFI ccall.

With this patch, the code

  {-# LANGUAGE MagicHash #-}
  module TestClz0 where
  import GHC.Prim
  ctz64 :: Word64# -> Word#
  ctz64 x = ctz64# x

results in the following assembler generated by NCG on i386:

  TestClz.ctz64_info:
      movl (%ebp),%eax
      movl 4(%ebp),%ecx
      movl %ecx,%edx
      orl %eax,%edx
      movl $64,%edx
      je _nAO

      bsf %ecx,%ecx
      addl $32,%ecx
      bsf %eax,%eax
      cmovne %eax,%ecx
      movl %ecx,%edx
  _nAO:
      movl %edx,%esi
      addl $8,%ebp
      jmp *(%ebp)

For comparision, here's what LLVM 3.4 currently generates:

  000000fc <TestClzz_ctzz64_info>:
    fc:   0f bc 45 04             bsf    0x4(%ebp),%eax
   100:   b9 20 00 00 00          mov    $0x20,%ecx
   105:   0f 45 c8                cmovne %eax,%ecx
   108:   83 c1 20                add    $0x20,%ecx
   10b:   8b 45 00                mov    0x0(%ebp),%eax
   10e:   8b 55 08                mov    0x8(%ebp),%edx
   111:   0f bc f0                bsf    %eax,%esi
   114:   85 c0                   test   %eax,%eax
   116:   0f 44 f1                cmove  %ecx,%esi
   119:   83 c5 08                add    $0x8,%ebp
   11c:   ff e2                   jmp    *%edx

Reviewed By: austin

Auditors: simonmar

Differential Revision: https://phabricator.haskell.org/D163