Can we support running pre ci on stacked PR?

[AllanZyne]

Is your feature request related to a problem? Please describe

Such as: #17180
If we support stacked PR workflow, we can make the code changes in each PR relatively small, which is good for code review and faster development.

Describe the solution you would like

Automatically trigger CI on PR that doesn't merge into "sycl" branch.

[aelovikov-intel]

IIUC, you're asking to remove this filtering:

llvm/.github/workflows/sycl-linux-precommit.yml

Lines 7 to 10 in a20c57d

	pull_request:
	branches:
	- sycl
	- sycl-rel-**

I'm not sure if that's a good idea. Maybe we have a convention that such PRs/branches should be named stacked-pr-**, then we can extend the filter (or maybe enable-precommit-**, or some other explicit pattern).

[AllanZyne]

It sounds good to have a PR naming pattern for stacked PRs.