[llvm] [AMDGPU] Exclude gfx1251 (w/ FullRate64Ops) from using GFX1250VALUBlockingCycles. (PR #226314)

[Nikhil Kotikalapudi via llvm-commits]

https://github.com/nkotikal commented:

PR description would be nice. 
Otherwise lgtm. 1251 is the only 1250 family subtarget where `hasFullRate64Ops()` is true right (for the sake of tests/robustness)?

https://github.com/llvm/llvm-project/pull/226314