i965/cfg: Point to bblock_t containing associated control flow

i965/cfg: Point to bblock_t containing associated control flow

... rather than pointing directly to the associated instruction. This
will let us set the block containing the IF statement's else-pointer to
NULL, when we delete a useless ELSE instruction, as in the case

   (+f0) if(8)
   ...
   else(8)
   endif(8)

Also, remove the pointer to the ENDIF, since it's unused, and it was
also potentially wrong, in the case of a basic block containing both an
ENDIF and an IF instruction:

   endif(8)
   cmp.ne.f0(8) ...
   (+f0) if(8)

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>