NTRU Prime: more unary negations

crypto_kem/sntrup653/clean/crypto_core_invsntrup653.c

@@ -15,7 +15,7 @@
 static int int16_nonzero_mask(int16 x) {
     uint16 u = (uint16) x; /* 0, else 1...65535 */
     uint32 v = u; /* 0, else 1...65535 */
-    v = -v; /* 0, else 2^32-65535...2^32-1 */
+    v = ~v + 1; /* 0, else 2^32-65535...2^32-1 */
     v >>= 31; /* 0, else 1 */
     return -(int) v; /* 0, else -1 */
 }

crypto_kem/sntrup761/clean/crypto_core_invsntrup761.c

@@ -15,7 +15,7 @@
 static int int16_nonzero_mask(int16 x) {
     uint16 u = (uint16) x; /* 0, else 1...65535 */
     uint32 v = u; /* 0, else 1...65535 */
-    v = -v; /* 0, else 2^32-65535...2^32-1 */
+    v = ~v + 1; /* 0, else 2^32-65535...2^32-1 */
     v >>= 31; /* 0, else 1 */
     return -(int) v; /* 0, else -1 */
 }

crypto_kem/sntrup857/clean/crypto_core_invsntrup857.c

@@ -15,7 +15,7 @@
 static int int16_nonzero_mask(int16 x) {
     uint16 u = (uint16) x; /* 0, else 1...65535 */
     uint32 v = u; /* 0, else 1...65535 */
-    v = -v; /* 0, else 2^32-65535...2^32-1 */
+    v = ~v + 1; /* 0, else 2^32-65535...2^32-1 */
     v >>= 31; /* 0, else 1 */
     return -(int) v; /* 0, else -1 */
 }