[llvm] [AMDGPU] Prefer `s_memtime` for `readcyclecounter` on GFX10 (PR #80211)

[via llvm-commits]

b-sumner wrote:

> > Umm...the cycle counter on gfx11 is 20 bits wide. I would oppose an attempt to implement the cycle counter with the realtime counter.
> 
> gfx10 still has this 64-bit version, but there's no alternative on gfx11. I don't know if gfx12 remedied this issue or not. Are you proposing gfx10 follows what gfx11 uses even when we have access to a higher resolution clock?

Sorry, I'd completely forgotten about that part of gfx10.  I agree that s_memtime should be used for readcyclecounter.

https://github.com/llvm/llvm-project/pull/80211