nir/gcm: Clear out pass_flags before starting

With this pass enabled in Intel drivers, running shader-db on
shaders/unity/38.shader_test resulted in

Program received signal SIGSEGV, Segmentation fault.
gcm_schedule_early_src (src=0x555555d45348, void_state=0x7fffffffba40) at ../../SOURCE/master/src/compiler/nir/nir_opt_gcm.c:297
297	   if (info->early_block->index < src_info->early_block->index)
(gdb) print src_info->early_block
$1 = (nir_block *) 0x0

I tracked this down to an early exit from gcm_schedule_early_instr on
the parent instruction because instr->pass_flags was 0x1c.  That
should be an impossible value for this pass, so I inferred that
pass_flags must have dirt left from some previous pass.

Fixes: 8dfe6f672f ("nir/GCM: Use pass_flags instead of bitsets for tracking visited/pinned")

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/597>
(cherry picked from commit 436668874a)

.pick_status.json

@@ -58,7 +58,7 @@
         "description": "nir/gcm: Clear out pass_flags before starting",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "8dfe6f672f4f3e226089c6cc8d392fcd39dff5cb"
     },

src/compiler/nir/nir_opt_gcm.c

@@ -604,6 +604,11 @@ opt_gcm_impl(nir_function_impl *impl, bool value_number)
    nir_metadata_require(impl, nir_metadata_block_index |
                               nir_metadata_dominance);
 
+   /* A previous pass may have left pass_flags dirty, so clear it all out. */
+   nir_foreach_block(block, impl)
+      nir_foreach_instr(instr, block)
+         instr->pass_flags = 0;
+
    struct gcm_state state;
 
    state.impl = impl;