[CIR][CodeGen] Handling multiple stmt followed after a switch case (llvm#878)

Close llvm#876

We've already considered the case that there are random stmt after a
switch case:

```
for (auto *c : compoundStmt->body()) {
      if (auto *switchCase = dyn_cast<SwitchCase>(c)) {
        res = buildSwitchCase(*switchCase, condType, caseAttrs);
      } else if (lastCaseBlock) {
        // This means it's a random stmt following up a case, just
        // emit it as part of previous known case.
        mlir::OpBuilder::InsertionGuard guardCase(builder);
        builder.setInsertionPointToEnd(lastCaseBlock);
        res = buildStmt(c, /*useCurrentScope=*/!isa<CompoundStmt>(c));
      } else {
        llvm_unreachable("statement doesn't belong to any case region, NYI");
      }

      lastCaseBlock = builder.getBlock();

      if (res.failed())
        break;
}
```

However, maybe this is an oversight, in the branch of ` if
(lastCaseBlock)`, the insertion point will be updated automatically when
the RAII object `guardCase` destroys, then we can assign the correct
value for `lastCaseBlock` later. So we will see the weird code pattern
in the issue side.

BTW, I found the codes in CIRGenStmt.cpp are far more less similar with
the ones other code gen places. Is this intentional? And what is the
motivation and guide lines here?

Author: ChuanqiXu9

clang/lib/CIR/CodeGen/CIRGenStmt.cpp

@@ -996,18 +996,18 @@ mlir::LogicalResult CIRGenFunction::buildSwitchBody(
     for (auto *c : compoundStmt->body()) {
       if (auto *switchCase = dyn_cast<SwitchCase>(c)) {
         res = buildSwitchCase(*switchCase, condType, caseAttrs);
+        lastCaseBlock = builder.getBlock();
       } else if (lastCaseBlock) {
         // This means it's a random stmt following up a case, just
         // emit it as part of previous known case.
         mlir::OpBuilder::InsertionGuard guardCase(builder);
         builder.setInsertionPointToEnd(lastCaseBlock);
         res = buildStmt(c, /*useCurrentScope=*/!isa<CompoundStmt>(c));
+        lastCaseBlock = builder.getBlock();
       } else {
         llvm_unreachable("statement doesn't belong to any case region, NYI");
       }
 
-      lastCaseBlock = builder.getBlock();
-
       if (res.failed())
         break;
     }

clang/test/CIR/CodeGen/goto.cpp

@@ -288,3 +288,25 @@ void foo() {
 // NOFLAT:   cir.scope {
 // NOFLAT:     cir.label "label"
 // NOFLAT:     %0 = cir.alloca !ty_S, !cir.ptr<!ty_S>, ["agg.tmp0"]
+
+extern "C" void action1();
+extern "C" void action2();
+extern "C" void multiple_non_case(int v) {
+  switch (v) {
+    default:
+        action1();
+      l2:
+        action2();
+        break;
+  }
+}
+
+// NOFLAT: cir.func @multiple_non_case
+// NOFLAT: cir.switch
+// NOFLAT: case (default)
+// NOFLAT: cir.call @action1()
+// NOFLAT: cir.br ^[[BB1:[a-zA-Z0-9]+]]
+// NOFLAT: ^[[BB1]]:
+// NOFLAT: cir.label
+// NOFLAT: cir.call @action2()
+// NOFLAT: cir.break