gpu: fix bugs with MemFence, Flat Instrs and Resource utilization

gpu: fix bugs with MemFence, Flat Instrs and Resource utilization

Both Memory Fence is now flagged as Global Memory only to avoid resource
oversubscribing.
Flat instructions now check for Shared Memory resource busy to avoid
oversubscribing resources.
All WaitClass resources now use cycles (not ticks) to register the number
of pipe stages between Scoreboard and Execute to be consistent with
instruction scheduling logic which always used clock cycles.