This commit moves issue select logic in reservation stations to stage 0
from stage 1. It helps timing of stage 1, which load-to-load requires.

Now, reservation stations have the following stages:

* S0: enqueue and wakeup, select. Selection results are RegNext-ed.
* S1: data/uop read and data bypass. Bypassed results are RegNext-ed.
* S2: issue instructions to function units.