lima/ppir: rework store output

lima/ppir: rework store output

In many cases, it is possible to avoid creating a mov for the store
output node.
Additionally, nodes other than alu, such as load varying, can be valid
store output nodes too.

This is another small optimization, but helps a vast majority of
programs by 1 instruction.
Shaders with discard easily become complicated to handle properly.
Some example issues: ppir has to rely on instruction ordering; or a
node with ssa output could be required both before a discard_if (as a
condition) and after it (as the instruction with the 'stop' bit set).
So don't try to handle them here.

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/4632>