[llvm] [AMDGPU] Add lit() asm operand modifier for SP3 compatibility. (PR #68839)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> > > abs(neg(x)) could be treated as abs(x).
> > 
> > 
> > So far I was under impression that these modifiers are supposed to designate corresponding encoding bits, just as our `id[:value]` instruction modifiers do, in which case the expectation would probably be to not allow combinations that are not supported by the target hardware.
> > Otherwise, if we see these more like generic expressions, then maybe using the good old minus sign would make more sense than `neg()`?
> 
> Fair point. I am happy for abs(neg(x)) to be an assembler error.

Technically -123.0 and neg(123.0) are different things. First is a negative constant. Second is a positive constant and a modifier. But noone does it, not even SP3.

https://github.com/llvm/llvm-project/pull/68839