Fix OMP CAS expansion with separate condition

When forcing the condition to be split out from COND_EXPRs I see
a runtime failure of libgomp.fortran/atomic-19.f90 which can be
reduced to

  !$omp atomic update, compare, capture
  if (x == 69_2 - r) x = 6_8
    v = x

being miscompiled, the difference being

-  _13 = .ATOMIC_COMPARE_EXCHANGE (_9, _10, _11, 4, 0, 0);
-  _14 = IMAGPART_EXPR <_13>;
-  _15 = REALPART_EXPR <_13>;
-  _16 = _14 != 0 ? _11 : _15;
-  _2 = (integer(kind=4)) _16;
-  v_17 = _2;
+  _14 = .ATOMIC_COMPARE_EXCHANGE (_10, _11, _12, 4, 0, 0);
+  _15 = IMAGPART_EXPR <_14>;
+  _16 = REALPART_EXPR <_14>;
+  _2 = (logical(kind=1)) _15;
+  _3 = (integer(kind=4)) _16;
+  v_17 = _3;

where one can see a missing COND_EXPR.  It seems to be a latent
issue to me given the code can be exercised, it just maybe misses
a 'need_new' testcase combined with 'cond_stmt'.  Appearantly
the if (cond_stmt) code is just to avoid creating a temporary
(and possibly to preserve the condition compute if used elsewhere
since the original stmt is going to be deleted).  The following
makes the failure go away for me in my patched tree and it
also survives libgomp and gomp testing in an unpatched tree.

2022-05-13  Richard Biener  <rguenther@suse.de>

	* omp-expand.cc (expand_omp_atomic_cas): Do not short-cut
	computation of the new value.