Fix interrupt hang

The thread select stage doens't handle clearing scoreboard entries for
instructions in the multicycle (fp) execution pipeline (it does for the
memory pipeline, and the logic is similar).  For now, just disable having
an interrupt occur when a multi-cycle instruction arrives at the writeback
stage.
Optimize delay in operand fetch stage.

This stage now has two internal stages: the first to read register SRAM
memory and the second to select the appropriate output signals.