Refactor subpattern logic out of Context (#4929)

Note this is building on #4927 which factors out RegionStack, because
this heavily uses the region stack. However, it's a more complex level
of logic that appears specific to pattern handling.

I've moved InsertHere to pattern_match.cpp because it appears to be in
specific use there.

toolchain/check/subpattern.cpp

@@ -0,0 +1,43 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/subpattern.h"
+
+namespace Carbon::Check {
+
+auto BeginSubpattern(Context& context) -> void {
+  context.inst_block_stack().Push();
+  context.region_stack().PushRegion(context.inst_block_stack().PeekOrAdd());
+}
+
+auto EndSubpatternAsExpr(Context& context, SemIR::InstId result_id)
+    -> SemIR::ExprRegionId {
+  if (context.region_stack().PeekRegion().size() > 1) {
+    // End the exit block with a branch to a successor block, whose contents
+    // will be determined later.
+    context.AddInst(SemIR::LocIdAndInst::NoLoc<SemIR::Branch>(
+        {.target_id = context.inst_blocks().AddDefaultValue()}));
+  } else {
+    // This single-block region will be inserted as a SpliceBlock, so we don't
+    // need control flow out of it.
+  }
+  auto block_id = context.inst_block_stack().Pop();
+  CARBON_CHECK(block_id == context.region_stack().PeekRegion().back());
+
+  // TODO: Is it possible to validate that this region is genuinely
+  // single-entry, single-exit?
+  return context.sem_ir().expr_regions().Add(
+      {.block_ids = context.region_stack().PopRegion(),
+       .result_id = result_id});
+}
+
+auto EndSubpatternAsEmpty(Context& context) -> void {
+  auto block_id = context.inst_block_stack().Pop();
+  CARBON_CHECK(block_id == context.region_stack().PeekRegion().back());
+  CARBON_CHECK(context.region_stack().PeekRegion().size() == 1);
+  CARBON_CHECK(context.inst_blocks().Get(block_id).empty());
+  context.region_stack().PopAndDiscardRegion();
+}
+
+}  // namespace Carbon::Check