nir/opt_remove_phis: skip unreachable phis

block->imm_dom is NULL for unreachable phis, so the dominance checks would crash.
These blocks should be removed by nir_opt_dead_cf, so don't bother optimizing
them here.

Fixes: 60776f87c3 ("nir/opt_remove_phis: rematerialize constants")
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35935>
(cherry picked from commit fcc9203550)

.pick_status.json

@@ -3144,7 +3144,7 @@
         "description": "nir/opt_remove_phis: skip unreachable phis",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "60776f87c38f69507d60591b46b3ea2efba8e188",
         "notes": null

src/compiler/nir/nir_opt_remove_phis.c

@@ -95,6 +95,10 @@ remove_phis_instr(nir_builder *b, nir_phi_instr *phi, void *unused)
    nir_def *def = NULL;
    bool needs_remat = false;
 
+   /* Skip unreachable phis, they should be removed by nir_opt_dead_cf. */
+   if (nir_block_is_unreachable(block))
+      return false;
+
    nir_foreach_phi_src(src, phi) {
       /* For phi nodes at the beginning of loops, we may encounter some
        * sources from backedges that point back to the destination of the