lima/ppir: remove orphan load node after cloning

lima/ppir: remove orphan load node after cloning

There are some cases in shades using control flow where the varying load
is cloned to every block, and then the original node is left orphan.
This is not harmful for program execution, but it complicates analysis
for register allocation as there is now a case of writing to a register
that is never read.
While ppir doesn't have a dead code elimination pass for its own
optimizations and it is not hard to detect when we cloned the last load,
let's remove it early.

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/3358>