[llvm] [AMDGPU] Classify FLAT instructions as VMEM (PR #137148)

[Jay Foad via llvm-commits]

jayfoad wrote:

_If_ we're going to do this then it should be done as an NFC change, so there should be no changes to lit tests.

If you discover anything that's currently broken due to isVMEM not including FLAT, then that should be fixed first in a separate patch.

https://github.com/llvm/llvm-project/pull/137148