The Vega ISA's s_memtime instruction is used to obtain a cycle value
from the GPU. Previously, this was implemented to obtain the cycle count
when the memtime instruction reached the execute stage of the GPU
pipeline. However, from microbenchmarking we have found that this under
reports the latency for memtime instructions relative to real hardware.
Thus, we changed its behavior to go through the scalar memory pipeline
and obtain a latency value from the the SQC (L1 I$). This mirrors the
suggestion of the AMD Vega ISA manual that s_memtime should be treated
like a s_load_dwordx2.
The default latency was set based on microbenchmarking.
Change-Id: I5e251dde28c06fe1c492aea4abf9f34f05784420
a8447b7fc0