i965: Safely iterate the predecessors of the end block.

i965: Safely iterate the predecessors of the end block.

We want to insert code in each of the predecessors of the end block.
This code includes a nir_if, which would split the block, altering
the set.  To avoid that, I emitted a dead constant at the end of each
block before splitting it, so that the set of predecessors remained
unchanged.  This was admittedly ugly.

Connor suggested instead saving a copy of the set, so we can iterate
it safely.  This is also a little ugly, but a much better plan.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>