vulkan: fuse mul_mat+add and mul_mat_id+add_id (#16868)
* vulkan: fuse mul_mat+add and mul_mat_id+add_id
The fusion is only applied for the mat-vec mul paths.
* Apply suggestions from code review
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
* fix 32b build
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
