shader: ignore usage mask when syncing layouts and searching IO slot
Since we no longer emit enhanced layouts IO slots can no longer overlap and we don't have to check the usage mask when searching for the IO slot.
This fixes picking up stale IO info that may be passed in with the shader key when it is initialized using an unrelated prev shader stage that might still be bound.
Fixes: 028542a2 shader: factor out input search and simplify code