broadcom/compiler: fix DAG pre-remove for merged instructions

broadcom/compiler: fix DAG pre-remove for merged instructions

When selecting an instruction to merge, we want to pre-remove that
instruction from the DAG, not the one we are merging it in, which
we had already pre-removed right before.

The reason this was not causing problems before is that the
consequence of this bug is we will choose the same instruction
again in the merge loop and trying to merge that instruction twice
will fail and we would break out of the merge loop and move on.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9304>