Change precision/scale truncate in decimal divide analysis.

Previously, we tried to maintain as much of the scale as possible but
this leads to very easy overflow cases since it requires dropping all
digits before the decimal point. This patch picks a midway point.

I did a little bit of research this is close to what SQL server does
(the reference is linked in the function I changed).

Change-Id: I2100beead82559ef7b017c5f335acd532076c0d4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3142
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins