broadcom/compiler: apply payload conflict to spill setup before RA

We can emit spill setup before RA if we use scratch. In that case
we have the same situation as during spilling, with the caveat that
we have already emitted the instructions so we need to find them
(they should be the only instructions ones before the instructions
accessing payload registers) and flag them as such.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
cc: mesa-stable

Part-of: <!29343>