compiler: Check stability with an accumulator wider than the field

The stability check sums the whole field and asks whether the result is
finite. The accumulator took the field's own dtype, so in half precision
it overflowed within a few thousand points and reported an instability
that wasn't there -- making `errctl=max`, the very option one reaches for
to diagnose a suspected instability, unusable exactly where it is needed.

Give it at least single precision.

Author: mloubout

devito/passes/iet/errors.py

@@ -56,7 +56,12 @@ def _check_stability(iet, wmovs=(), rcompile=None, sregistry=None):
         else:
             continue
 
-        accumulator = Symbol(name='accumulator', dtype=f.dtype)
+        # The accumulator sums the whole field, so it is given at least single
+        # precision: in half precision it would overflow within a few thousand
+        # points and report an instability that isn't there
+        dtype = np.promote_types(f.dtype, np.float32).type
+
+        accumulator = Symbol(name='accumulator', dtype=dtype)
         eqns = [Eq(accumulator, 0.0),
                 Inc(accumulator, f.subs(f.time_dim, 0))]
         irs, byproduct = rcompile(eqns)