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_if_statement.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"
 
@@ -22,8 +23,8 @@ auto HandleParseNode(Context& context, Parse::IfConditionId node_id) -> bool {
   // there is no `else`, the then block will terminate with a branch to the
   // else block, which will be reused as the resumption block.
   auto then_block_id =
-      context.AddDominatedBlockAndBranchIf(node_id, cond_value_id);
-  auto else_block_id = context.AddDominatedBlockAndBranch(node_id);
+      AddDominatedBlockAndBranchIf(context, node_id, cond_value_id);
+  auto else_block_id = AddDominatedBlockAndBranch(context, node_id);
 
   // Start emitting the `then` block.
   context.inst_block_stack().Pop();
@@ -64,7 +65,7 @@ auto HandleParseNode(Context& context, Parse::IfStatementId node_id) -> bool {
       // Branch from the then and else blocks to a new resumption block.
       context.node_stack()
           .PopAndDiscardSoloNodeId<Parse::NodeKind::IfStatementElse>();
-      context.AddConvergenceBlockAndPush(node_id, /*num_blocks=*/2);
+      AddConvergenceBlockAndPush(context, node_id, /*num_blocks=*/2);
       break;
     }