[llvm] [AMDGPU] Remove FlatVariant argument from isLegalFlatAddressingMode. NFC. (PR #93938)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> It's not a 1:1 mapping though, you can select global addresses to flat instructions (which the patterns do handle)

But every single use is 1:1.

https://github.com/llvm/llvm-project/pull/93938