JIT: Use rsb and shift in easy multiply.

For easy multiplication using reverse subtract (when
lit is 2^n-1) use the barrel shifter for rsb.

This improves arithmetic performance for code executing
in Dalvik. E.g String.hashCode.

Change-Id: Ifb086dcec344b30fd3e392ac21d508b43e820cdc
Signed-off-by: Patrik Ryd <patrik.ryd@stericsson.com>