[X86] Dont run combineSetCCAtomicArith() when the cmp has multiple uses

[X86] Dont run combineSetCCAtomicArith() when the cmp has multiple uses

We would miscompile the following:

  void g(int);
  int f(volatile long long *p) {
    bool b = __atomic_fetch_add(p, 1, __ATOMIC_SEQ_CST) < 0;
    g(b ? 12 : 34);
    return b ? 56 : 78;
  }

into

  pushq   %rax
  lock            incq    (%rdi)
  movl    $12, %eax
  movl    $34, %edi
  cmovlel %eax, %edi
  callq   g(int)
  testq   %rax, %rax   <---- Bad.
  movl    $56, %ecx
  movl    $78, %eax
  cmovsl  %ecx, %eax
  popq    %rcx
  retq

because the code failed to take into account that the cmp has multiple
uses, replaced one of them, and left the other one comparing garbage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291630 91177308-0d34-0410-b5e6-96231b3b80d8