[llvm] [AMDGPU] Add FeatureGFX13 and SMEM encoding for gfx13 (PR #177567)

[Mariusz Sikora via llvm-commits]

mariusz-sikora-at-amd wrote:

> hasGFX1250Insts isn't quite the same but the real problem is using these catch-all named checks instead of something more specific

So maybe I will just add to `GCNSubtarget.h`
```
bool isGFX1250() { return HasGFX1250Insts && !HasGFX13Insts; }
bool isGFX1250Plus() { return HasGFX1250Insts; }
```

https://github.com/llvm/llvm-project/pull/177567