(multcosts): Do not compute cost of constant power-of-two multiplies as if they were...

(multcosts): Do not compute cost of constant power-of-two multiplies as if they were shifts.

(multcosts): Do not compute cost of constant power-of-two
multiplies as if they were shifts.  Reduce multiply cost from 5 to 3.
Delete variable insn_cost.

From-SVN: r8777