[mlir][gpu] Split ops sinking from gpu-kernel-outlining pass into separate pass

Previously `gpu-kernel-outlining` pass was also doing index computation sinking into gpu.launch before actual outlining.
Split ops sinking from `gpu-kernel-outlining` pass into separate pass, so users can use theirs own sinking pass before outlining.
To achieve old behavior users will need to call both passes: `-gpu-launch-sink-index-computations -gpu-kernel-outlining`.

Differential Revision: https://reviews.llvm.org/D119932