[X86] Emit more efficient >= comparisons against 0

We don't do a great job with >= 0 comparisons against zero when the
result is used as an i8.

Given something like:
  void f(long long LL, bool *B) {
    *B = LL >= 0;
  }

We used to generate:
  shrq    $63, %rdi
  xorb    $1, %dil
  movb    %dil, (%rsi)

Now we generate:
  testq   %rdi, %rdi
  setns   (%rsi)

Differential Revision: http://reviews.llvm.org/D12136

llvm-svn: 245498