First cl to implement a pass to unstack loop operands.

This pass implements unstacking for loop operands. Generally speaking, unstacking is the act of breaking a rank n tensor into n smaller n-1 rank tensors without changing the semantics of the program. There are different patterns that can benefit from unstacking. This pass aims to implement such patterns. The patterns implemented are not exhaustive by any means. There are more patterns to be added.
The pass is not added to the compiler yet.

PiperOrigin-RevId: 638785310

xla/service/BUILD

@@ -3248,6 +3248,44 @@ xla_cc_test(
     ],
 )
 
+cc_library(
+    name = "hlo_unstacker",
+    srcs = ["hlo_unstacker.cc"],
+    hdrs = ["hlo_unstacker.h"],
+    deps = [
+        ":hlo_creation_utils",
+        ":hlo_pass",
+        ":pattern_matcher",
+        ":tuple_util",
+        ":while_loop_unroller",
+        "//xla:shape_util",
+        "//xla:util",
+        "//xla/hlo/ir:hlo",
+        "@com_google_absl//absl/container:flat_hash_map",
+        "@com_google_absl//absl/container:flat_hash_set",
+        "@com_google_absl//absl/log",
+        "@com_google_absl//absl/log:check",
+        "@com_google_absl//absl/status",
+        "@com_google_absl//absl/status:statusor",
+        "@com_google_absl//absl/strings:string_view",
+        "@tsl//tsl/platform:errors",
+        "@tsl//tsl/platform:statusor",
+    ],
+)
+
+xla_cc_test(
+    name = "hlo_unstacker_test",
+    srcs = ["hlo_unstacker_test.cc"],
+    tags = ["requires-net:external"],
+    deps = [
+        ":hlo_unstacker",
+        "//xla/hlo/ir:hlo",
+        "//xla/tests:hlo_test_base",
+        "@com_google_googletest//:gtest_main",
+        "@tsl//tsl/platform:statusor",
+    ],
+)
+
 cc_library(
     name = "while_loop_unroller",
     srcs = ["while_loop_unroller.cc"],