[mlir] Support pre-existing tokens in 'gpu-async-region'

Allow gpu ops implementing the async interface to already be async when running the GpuAsyncRegionPass.
That pass threads a 'current token' through a block with ops implementing the gpu async interface.

After this change, existing async ops (returning a !gpu.async.token) set the current token.
Existing synchronous `gpu.wait` ops reset the current token.

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D103396