5   Expressions                                                 [expr]

 [...]
9 Many binary operators that expect operands of arithmetic  or  enumera-
  tion  type  cause conversions and yield result types in a similar way.
  The purpose is to yield a common type, which is also the type  of  the
  result.   This  pattern  is  called  the usual arithmetic conversions,
  which are defined as follows:

 [...]
 4.5  Integral promotions                                   [conv.prom]

1 An  rvalue  of  type  char,  signed char, unsigned char, short int, or
  unsigned short int can be converted to an rvalue of type  int  if  int
  can represent all the values of the source type; otherwise, the source
  rvalue can be converted to an rvalue of type unsigned int.

Короче говоря, в этом выражении сначала считается x[1] << 8. Поскольку 8 имеет тип int, то и выражение должно иметь тип int. Далее результат int прибавляется к x[0], в итоге тоже должен получиться int.