[XLA:GPU] track runtime variables in indexing analysis
For generic triton emitter we currenly lack information about all values that are needed. For example, for `dynamic-slice(data, offset)` to properly load `data` we need to have `offset` value present (and already emitted), as `data` indirectly depends on `offset` by participating in dynamic slice.
That change adds runtime variables as part of OperandIndexing (instead of just plain indexing map). They will be used to generate tiled instructions that also reference runtime counterparts.
PiperOrigin-RevId: 767058843
