Optimize more easy multiplications by constants.

Optimize more easy multiplications by constants.

Rather than make these changes in the libraries (*10 being a common case),
let's do them once and for all in the JIT.

The 2^n-1 case could be better if we generated RSB instructions, but the
current "fake" RSB is still better than a full multiply.

Thumb doesn't support reg/reg/reg/shift instructions, so we can't optimize
the "population count <= 2" cases (such as *10) there.

Tested on sholes, passion, and passion-running-sapphire (and visually
inspected to check we weren't trying to generate Thumb2 instructions there).
Also tested with the self-verifier.