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

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`.

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D147837

Author: aheejin

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

@@ -637,10 +637,10 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
       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);

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

@@ -221,13 +221,20 @@ drop_empty_stack_while_popping:
   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