pan/bi: Fix source swizzle in bi_repair_ssa

Repairing SSA was creating invalid PHI nodes with source swizzles !=
BI_SWIZZLE_H01. PHI sources can't have non-identity swizzles.

In most cases the repair logic only replaces sources, in which case the
swizzle is taken from the old source that is getting replaced. However,
in add_phi_operands there is no old source because the phi is new, and
so the result from resolve_read is assigned directly. This falsely
carries over the destination swizzle to the source.

Since it never makes sense for resolve_read to carry over the swizzle
from the instruction writing the value, we can make it so that
resolve_read always returns the identity swizzle on indices.
resolve_read returns one of:
- An index stored by record_write
- An index created by bi_temp_like
- The result of a recursive resolve_read call
bi_temp_like already correctly sets the swizzle to H01. Setting it in
record_write leads to both base cases returning the desired swizzle.

Fixes: dd94d183 ("pan/bi: Fixup bi_repair_ssa.c for bi")
Reviewed-by: Lorenzo Rossi <lorenzo.rossi@collabora.com>
Reviewed-by: Eric R. Smith <eric.smith@collabora.com>
(cherry picked from commit fcfc580f67)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41268>

.pick_status.json

@@ -2344,7 +2344,7 @@
         "description": "pan/bi: Fix source swizzle in bi_repair_ssa",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "dd94d1833f6cd05a123ca4aa038a6547685ec20b",
         "notes": null

src/panfrost/compiler/bifrost/bi_repair_ssa.c

@@ -50,6 +50,14 @@ record_write(struct repair_ctx *ctx, bi_block *block, unsigned node,
              bi_index val)
 {
    assert(node < ctx->n);
+
+   /* The value always gets used as a source, where we don't want the
+    * destination "swizzle" and other modifiers that are applied on write.
+    */
+   val.swizzle = BI_SWIZZLE_H01;
+   val.neg = false;
+   val.abs = false;
+
    struct hash_table_u64 *defs = repair_block(ctx, block)->defs;
    _mesa_hash_table_u64_insert(defs, node,
                                ralloc_memdup(defs, &val, sizeof(val)));
@@ -106,6 +114,7 @@ resolve_read(struct repair_ctx *ctx, bi_block *block, bi_index node)
    }
 
    assert(!bi_is_null(val));
+   assert(val.swizzle == BI_SWIZZLE_H01);
    record_write(ctx, block, node.value, val);
    return val;
 }