[llvm] [AMDGPU][AsmParser][NFC] Give isImmLiteral() a better name. (PR #153395)

[Mirko Brkušanin via llvm-commits]

mbrkusanin wrote:

I agree that old name `isImmLiteral` could be misleading.

`ImmTyPlain` is also strange to me. I find old `ImmTyNone` less confusing or maybe `ImmTyRegular`.

https://github.com/llvm/llvm-project/pull/153395