VALU instructions should take 4 cycles rather than 5 cycles.

[syifan]

To Reproduce
MGPUSim version of commit ID: 7810efc

Command that recreates the problem

./fir -timing --trace-vis

Current behavior
It takes 5 cycles for VALU instructions to execute. And because of the round-robin scheduler, the gap between two instructions are 8 cycles.

Expected behavior
If there is only one wavefront running in one CU and there is only VALU instructions, VALU instructions should lay back to back.