[llvm] [AMDGPU] Select the GFX80 encoding family from the subtarget. NFCI. (PR #219190)

[Jay Foad via llvm-commits]

jayfoad wrote:

Ping!

The motivation here is to get rid of some of the weirder special cases in pseudoToMCOpcode. The remaining special cases do not depend on the opcode, they are just blanket fallbacks from one EncodingFamily to another, and I have a plan to unify all of those.

https://github.com/llvm/llvm-project/pull/219190