Avoid unnecessary pointer dereferencing.

Avoid unnecessary pointer dereferencing.

- modulo_shift is an unsigned value
- do not hardcode builtin mode_Iu, it may not be the correct mode
- avoid potentially huge values in new_tarval_from_long

Initialize mode values directly. This is to avoid users using
get_tarval_XXX() instead of get_mode_XXX() which is always preferable.

- Do not use predefined mode_Iu, it's not necessary the frontends
  unsigned mode!
- tarvals are identified in a hashmap, we can compare their pointers for
  equality
- modulo_shift is an unsigned value, not an int

If a mode has modulo_shift == 2**n, we only care about the n least
significant bits of the shift amount.