Move mbedtls_cf_size_bool_eq function to the constant-time module

There were multiple functions called mbedtls_cf_size_bool_eq. They had exactly
the same behavior, so move the one in bignum.c and remove the other.

Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>

library/constant_time.c

@@ -177,3 +177,39 @@ size_t mbedtls_cf_size_mask_ge( size_t x, size_t y )
 {
     return( ~mbedtls_cf_size_mask_lt( x, y ) );
 }
+
+/*
+ * Constant-flow boolean "equal" comparison:
+ * return x == y
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations - it can be used in conjunction with
+ * mbedtls_ssl_cf_mask_from_bit().
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+size_t mbedtls_cf_size_bool_eq( size_t x, size_t y )
+{
+    /* diff = 0 if x == y, non-zero otherwise */
+    const size_t diff = x ^ y;
+
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+    /* diff_msb's most significant bit is equal to x != y */
+    const size_t diff_msb = ( diff | (size_t) -diff );
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    /* diff1 = (x != y) ? 1 : 0 */
+    const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
+
+    return( 1 ^ diff1 );
+}