nir/phi_builder: Don't recurse in value_get_block_def

nir/phi_builder: Don't recurse in value_get_block_def

In some programs, we can have very deep dominance trees and the recursion
can cause us to risk stack overflows.  Instead, we replace the recursion
with a pair of loops, one at the start and one at the end.  This is
functionally equivalent to what we had before and it's actually a bit
easier to read in the new form without the recursion.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97225
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>