[X86] Fold (setcc (cmp (atomic_load_add x, -C) C), COND) to (setcc (LADD x, -C), COND) (PR31367)

atomic_load_add returns the value before addition, but sets EFLAGS based on the
result of the addition. That means it's setting the flags based on effectively
subtracting C from the value at x, which is also what the outer cmp does.

This targets a pattern that occurs frequently with reference counting pointers:

  void decrement(long volatile *ptr) {
    if (_InterlockedDecrement(ptr) == 0)
      release();
  }

Clang would previously compile it (for 32-bit at -Os) as:

00000000 <?decrement@@YAXPCJ@Z>:
   0:   8b 44 24 04             mov    0x4(%esp),%eax
   4:   31 c9                   xor    %ecx,%ecx
   6:   49                      dec    %ecx
   7:   f0 0f c1 08             lock xadd %ecx,(%eax)
   b:   83 f9 01                cmp    $0x1,%ecx
   e:   0f 84 00 00 00 00       je     14 <?decrement@@YAXPCJ@Z+0x14>
  14:   c3                      ret

and with this patch it becomes:

00000000 <?decrement@@YAXPCJ@Z>:
   0:   8b 44 24 04             mov    0x4(%esp),%eax
   4:   f0 ff 08                lock decl (%eax)
   7:   0f 84 00 00 00 00       je     d <?decrement@@YAXPCJ@Z+0xd>
   d:   c3                      ret

(Equivalent variants with _InterlockedExchangeAdd, std::atomic<>'s fetch_add
or pre-decrement operator generate the same code.)

Differential Revision: https://reviews.llvm.org/D27781

llvm-svn: 289955