panfrost/midgard: Fix nested/chained if-else

panfrost/midgard: Fix nested/chained if-else

An if-else statement is compiled to a conditional branch (from the start
to the second block) and an unconditional branch (from the end of the
first block to the end of the else). We previously incorrectly computed
the block index of the unconditional branch to be exactly one after that
of the conditional branch, valid for a single if-else statement but
nothing fancier. This patch correctly computes the unconditional branch
target, fixing more complex if-else chains.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>