Move control flow block functions to their own file. (#4921)

context.cpp is getting large, so I'm looking at a few ways to cut out
clusters of functions. This felt like a logical cluster of functions to
move to their own file.

Note this doesn't touch the implementation at all, beyond what's needed
to change from `Context` members to context args.

toolchain/check/handle_operator.cpp

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include "toolchain/check/context.h"
+#include "toolchain/check/control_flow.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/handle.h"
 #include "toolchain/check/operator.h"
@@ -349,9 +350,9 @@ static auto HandleShortCircuitOperand(Context& context, Parse::NodeId node_id,
 
   // Create a block for the right-hand side and for the continuation.
   auto rhs_block_id =
-      context.AddDominatedBlockAndBranchIf(node_id, branch_value_id);
-  auto end_block_id = context.AddDominatedBlockAndBranchWithArg(
-      node_id, short_circuit_result_id);
+      AddDominatedBlockAndBranchIf(context, node_id, branch_value_id);
+  auto end_block_id = AddDominatedBlockAndBranchWithArg(
+      context, node_id, short_circuit_result_id);
 
   // Push the branch condition and result for use when handling the complete
   // expression.
@@ -410,8 +411,8 @@ static auto HandleShortCircuitOperator(Context& context, Parse::NodeId node_id)
   auto result_id = context.AddInst<SemIR::BlockArg>(
       node_id, {.type_id = context.insts().Get(rhs_id).type_id(),
                 .block_id = resume_block_id});
-  context.SetBlockArgResultBeforeConstantUse(result_id, branch_value_id, rhs_id,
-                                             short_circuit_result_id);
+  SetBlockArgResultBeforeConstantUse(context, result_id, branch_value_id,
+                                     rhs_id, short_circuit_result_id);
   context.node_stack().Push(node_id, result_id);
   return true;
 }