[Explicit State Access] Support multiple State Reads nodes for a single State Element in proc_state_legalization_pass

Prevent nullptr from attempting to fetch State Reads from Next Nodes constructed from a State Element.

#geminiassisted

PiperOrigin-RevId: 910694399

Author: NL02

xls/scheduling/BUILD

@@ -460,6 +460,7 @@ cc_library(
         "//xls/ir:value",
         "//xls/passes:pass_base",
         "//xls/solvers:z3_ir_translator",
+        "@com_google_absl//absl/algorithm:container",
         "@com_google_absl//absl/container:btree",
         "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/log",
@@ -488,7 +489,9 @@ cc_test(
         "//xls/ir:function_builder",
         "//xls/ir:ir_matcher",
         "//xls/ir:ir_test_base",
+        "//xls/ir:op",
         "//xls/ir:proc_conversion",
+        "//xls/ir:source_location",
         "//xls/ir:value",
         "//xls/passes:cse_pass",
         "//xls/passes:dce_pass",