lima/ppir: do not assume single src for pipeline outputs

lima/ppir: do not assume single src for pipeline outputs

Even if a node has pipeline output and a single successor, it is still
valid for that successor to have multiple references to that pipeline
node. A trivial example is add(u.x,u.y) where u is a uniform.
It is even possible for this to occur with consts as operands of fcsel.
So remove uses of ppir_node_get_src_for_pred as that would assume a
single src in the node that uses the pipeline.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4535>