nir: rewrite nir_foreach_block and friends

nir: rewrite nir_foreach_block and friends

Previously, these were functions which took a callback. This meant that
the per-block code had to be in a separate function, and all the data
that you wanted to pass in had to be a single void *. They walked the
control flow tree recursively, doing a depth-first search, and called
the callback in a preorder, matching the order of the original source
code. But since each node in the control flow tree has a pointer to its
parent, we can implement a "get-next" and "get-previous" method that
does the same thing that the recursive function did with no state at
all. This lets us rewrite nir_foreach_block() as a simple for loop,
which lets us greatly simplify its users in some cases. This does
require us to rewrite every user, although the transformation from the
old nir_foreach_block() to the new nir_foreach_block() is mostly
trivial.

One subtlety, though, is that the new nir_foreach_block() won't handle
the case where the current block is deleted, which the old one could.
There's a new nir_foreach_block_safe() which implements the standard
trick for solving this. Most users don't modify control flow, though, so
they won't need it. Right now, only opt_select_peephole needs it.

The old functions are reimplemented in terms of the new macros, although
they'll go away after everything is converted.

v2: keep an implementation of the old functions around
v3 (Jason Ekstrand): A small cosmetic change and a bugfix in the loop
   handling of nir_cf_node_cf_tree_last().
v4 (Jason Ekstrand): Use the _safe macro in foreach_block_reverse_call

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>