Add flag to control selection of default vulkan device.
Add flag to control selection of default vulkan device.

We'll need a better facility later, but for now, this lets me access both of my GPUs for testing.

Closes https://github.com/google/iree/pull/1676

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/1676 from stellaraccident:devsel 81127ce53290987f69e3cc756fc87c43cdca3b64
PiperOrigin-RevId: 308627841

google/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree