redefine VERIFY_CHECK to empty in production (non-VERIFY) mode

As suggested in issue bitcoin#1381, this will make things simpler and
improve code readability, as we don't need to force omitting of
evaluations on a case-by-case basis anymore and hence can remove
lots of `#ifdef VERIFY`/`#endif` lines (see next commit). Plus,
VERIFY_CHECK behaves now identical in both non-VERIFY and coverage mode,
making the latter not special anymore and hopefully decreasing
maintenance burden. The idea of "side-effect safety" is given up.

Note that at two places in the ellswift module void-casts of return
values have to be inserted for non-VERIFY builds, in order to avoid
   "variable ... set but not used [-Wunused-but-set-variable]"
warnings.

src/modules/ellswift/main_impl.h

@@ -272,7 +272,11 @@ static int secp256k1_ellswift_xswiftec_inv_var(secp256k1_fe *t, const secp256k1_
         secp256k1_fe_negate(&q, &q, 1);                 /* q = -s*(4*(u^3+7)+3*u^2*s) */
         if (!secp256k1_fe_is_square_var(&q)) return 0;
         ret = secp256k1_fe_sqrt(&r, &q);                /* r = sqrt(-s*(4*(u^3+7)+3*u^2*s)) */
+#ifdef VERIFY
         VERIFY_CHECK(ret);
+#else
+        (void)ret;
+#endif
 
         /* If (c & 1) = 1 and r = 0, fail. */
         if (EXPECT((c & 1) && secp256k1_fe_normalizes_to_zero_var(&r), 0)) return 0;
@@ -417,7 +421,11 @@ int secp256k1_ellswift_encode(const secp256k1_context *ctx, unsigned char *ell64
          * BIP340 tagged hash with tag "secp256k1_ellswift_encode". */
         secp256k1_ellswift_sha256_init_encode(&hash);
         ser_ret = secp256k1_eckey_pubkey_serialize(&p, p64, &ser_size, 1);
+#ifdef VERIFY
         VERIFY_CHECK(ser_ret && ser_size == 33);
+#else
+        (void)ser_ret;
+#endif
         secp256k1_sha256_write(&hash, p64, sizeof(p64));
         secp256k1_sha256_write(&hash, rnd32, 32);
 

src/util.h

@@ -132,15 +132,12 @@ static const secp256k1_callback default_error_callback = {
 } while(0)
 #endif
 
-/* Like assert(), but when VERIFY is defined, and side-effect safe. */
-#if defined(COVERAGE)
-#define VERIFY_CHECK(check)
-#define VERIFY_SETUP(stmt)
-#elif defined(VERIFY)
+/* Like assert(), but when VERIFY is defined. */
+#if defined(VERIFY)
 #define VERIFY_CHECK CHECK
 #define VERIFY_SETUP(stmt) do { stmt; } while(0)
 #else
-#define VERIFY_CHECK(cond) do { (void)(cond); } while(0)
+#define VERIFY_CHECK(cond)
 #define VERIFY_SETUP(stmt)
 #endif