[WebAssembly] Fix type index block type handling in type checker

[aheejin]

Summary

The current code is

ExpectBlockType = false;
TC.setLastSig(*Signature.get());
if (ExpectBlockType)
  NestingStack.back().Sig = *Signature.get();

Because of the first line, the third line's if (ExpectBlockType) is
always false and we don't get to update NestingStack.back().Sig. This
results in not correctly erroring out when the types of remaining values
on the stack do not match the block type if the block type is written in
the form of a function type. We should set ExpectBlockType to false
after the if.

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

if (ExpectFuncType || (ExpectBlockType && Lexer.is(AsmToken::LParen))) {
       // represent as a signature, such that we can re-build this signature,
       // attach it to an anonymous symbol, which is what WasmObjectWriter
       // expects to be able to recreate the actual unique-ified type indices.
       auto Loc = Parser.getTok();
       auto Signature = std::make_unique<wasm::WasmSignature>();
       if (parseSignature(Signature.get()))
         return true;
       // Got signature as block type, don't need more
-      ExpectBlockType = false;
       TC.setLastSig(*Signature.get());
       if (ExpectBlockType)
         NestingStack.back().Sig = *Signature.get();
+      ExpectBlockType = false;
       auto &Ctx = getContext();
       // The "true" here will cause this to be a nameless symbol.
       MCSymbol *Sym = Ctx.createTempSymbol("typeindex", true);
       auto *WasmSym = cast<MCSymbolWasm>(Sym);
       WasmSym->setSignature(Signature.get());
       addSignature(std::move(Signature));
       WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
       const MCExpr *Expr = MCSymbolRefExpr::create(

llvm/test/MC/WebAssembly/type-checker-errors.s

# CHECK: :[[@LINE+1]]:3: error: popped f32, expected i32
   end_function
 
 drop_empty_stack_while_popping:
   .functype drop_empty_stack_while_popping () -> ()
 # CHECK: :[[@LINE+1]]:3: error: empty stack while popping value
   drop
   end_function
 
-end_block_insufficient_values_on_stack:
-  .functype end_block_insufficient_values_on_stack () -> ()
+end_block_insufficient_values_on_stack_1:
+  .functype end_block_insufficient_values_on_stack_1 () -> ()
   block i32
 # CHECK: :[[@LINE+1]]:3: error: end: insufficient values on the type stack
   end_block
   end_function
 
+end_block_insufficient_values_on_stack_2:
+  .functype end_block_insufficient_values_on_stack_2 () -> ()
+  block () -> (i32)
+# CHECK: :[[@LINE+1]]:3: error: end: insufficient values on the type stack
+  end_block
+  end_function
+
 end_block_type_mismatch:
   .functype end_block_type_mismatch () -> ()
   block i32
   f32.const 1.0
 # CHECK: :[[@LINE+1]]:3: error: end got f32, expected i32
   end_block
   end_function