[multi-gpu] Phase 1: rename channel_type in cascade_chain_* tests

origin/main grew 5 new herd-placement tests via #1583 that use the
pre-rename `channel_type = "cascade"`. After this PR's namespace rename
("cascade" -> "npu_cascade"), those tests fail under air-opt with the
verifier rejecting the old name. Update them to "npu_cascade" so they
keep passing on top of phase 1.

Verified on rad-mi300a-sh5-1: AIRHerdPlacement 15/15 pass, Dialect/AIR
21/21 pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwei-xilinx

mlir/test/Transform/AIRHerdPlacement/cascade_chain_3herd.mlir

@@ -18,8 +18,8 @@
 // CHECK: air.herd @consumer   {{.*}} attributes {{{.*}}x_loc = 0 : i64, y_loc = 2 : i64}
 
 module {
-  air.channel @ab [8, 1] {channel_type = "cascade"}
-  air.channel @bc [8, 1] {channel_type = "cascade"}
+  air.channel @ab [8, 1] {channel_type = "npu_cascade"}
+  air.channel @bc [8, 1] {channel_type = "npu_cascade"}
 
   func.func @three_herd_cascade_chain() {
     %c1 = arith.constant 1 : index

mlir/test/Transform/AIRHerdPlacement/cascade_chain_3herd_ew.mlir

@@ -17,8 +17,8 @@
 // CHECK: air.herd @consumer   {{.*}} attributes {{{.*}}x_loc = 2 : i64, y_loc = 2 : i64}
 
 module {
-  air.channel @ab [1, 8] {channel_type = "cascade"}
-  air.channel @bc [1, 8] {channel_type = "cascade"}
+  air.channel @ab [1, 8] {channel_type = "npu_cascade"}
+  air.channel @bc [1, 8] {channel_type = "npu_cascade"}
 
   func.func @three_herd_cascade_chain_ew() {
     %c1 = arith.constant 1 : index

mlir/test/Transform/AIRHerdPlacement/cascade_chain_3herd_with_l1_upstream.mlir

@@ -22,8 +22,8 @@
 
 module {
   air.channel @upstream_to_a [1, 1] {broadcast_shape = [8 : index, 1 : index]}
-  air.channel @ab_q [8, 1] {channel_type = "cascade"}
-  air.channel @bc_q [8, 1] {channel_type = "cascade"}
+  air.channel @ab_q [8, 1] {channel_type = "npu_cascade"}
+  air.channel @bc_q [8, 1] {channel_type = "npu_cascade"}
 
   func.func @upstream_then_3_chain() {
     %c1 = arith.constant 1 : index

mlir/test/Transform/AIRHerdPlacement/cascade_chain_4herd.mlir

@@ -18,9 +18,9 @@
 // CHECK: air.herd @h3 {{.*}} attributes {{{.*}}x_loc = 0 : i64, y_loc = 0 : i64}
 
 module {
-  air.channel @c01 [8, 1] {channel_type = "cascade"}
-  air.channel @c12 [8, 1] {channel_type = "cascade"}
-  air.channel @c23 [8, 1] {channel_type = "cascade"}
+  air.channel @c01 [8, 1] {channel_type = "npu_cascade"}
+  air.channel @c12 [8, 1] {channel_type = "npu_cascade"}
+  air.channel @c23 [8, 1] {channel_type = "npu_cascade"}
 
   func.func @four_herd_cascade_chain() {
     %c1 = arith.constant 1 : index

mlir/test/Transform/AIRHerdPlacement/cascade_chain_multi_channel.mlir

@@ -19,12 +19,12 @@
 // CHECK: air.herd @consumer   {{.*}} attributes {{{.*}}x_loc = 0 : i64, y_loc = 2 : i64}
 
 module {
-  air.channel @ab_q [8, 1] {channel_type = "cascade"}
-  air.channel @ab_k [8, 1] {channel_type = "cascade"}
-  air.channel @ab_v [8, 1] {channel_type = "cascade"}
-  air.channel @bc_q [8, 1] {channel_type = "cascade"}
-  air.channel @bc_k [8, 1] {channel_type = "cascade"}
-  air.channel @bc_v [8, 1] {channel_type = "cascade"}
+  air.channel @ab_q [8, 1] {channel_type = "npu_cascade"}
+  air.channel @ab_k [8, 1] {channel_type = "npu_cascade"}
+  air.channel @ab_v [8, 1] {channel_type = "npu_cascade"}
+  air.channel @bc_q [8, 1] {channel_type = "npu_cascade"}
+  air.channel @bc_k [8, 1] {channel_type = "npu_cascade"}
+  air.channel @bc_v [8, 1] {channel_type = "npu_cascade"}
 
   func.func @three_herd_multi_channel() {
     %c1 = arith.constant 1 : index