(expand_mult): Generalize to call synth_mult also for OP1 - 1.

(ceil_log2): New function.
(choose_multiplier): New function.
(invert_mod2n): New function.
(expand_mult_highpart_adjust): New function.
(expand_mult_highpart): New function.
(EXACT_POWER_OF_2_OR_ZERO_P): New macro.
(expand_divmod): Almost completely rewritten.
(expand_shift): Don't truncate immediate shift count, it doesn't work
for types smaller than int.

From-SVN: r7598