[WebAssembly][MC] Fix missing `else` after `return` due to type checker bug

[sbc100]

Summary

Once we are in the Unreachable we want to disable type checking, but
we were unconditionally returning true here which means we encountered
and error. Instead we unconditionally return false to signal no error.

Fixes: https://github.com/llvm/llvm-project/issues/56935

[dschuff]

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

77

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

void WebAssemblyAsmTypeCheck::dumpTypeStack(Twine Msg) {
   });
 }
 
 bool WebAssemblyAsmTypeCheck::typeError(SMLoc ErrorLoc, const Twine &Msg) {
   // Once you get one type error in a function, it will likely trigger more
   // which are mostly not helpful.
   if (TypeErrorThisFunction)
     return true;
-  // If we're currently in unreachable code, we surpress errors as well.
+  // If we're currently in unreachable code, we surpress errors completely.
   if (Unreachable)
-    return true;
+    return false;
   TypeErrorThisFunction = true;
   dumpTypeStack("current stack: ");
   return Parser.Error(ErrorLoc, Msg);
 }
 
 bool WebAssemblyAsmTypeCheck::popType(SMLoc ErrorLoc,
                                       Optional<wasm::ValType> EVT) {
   if (Stack.empty()) {

llvm/test/MC/WebAssembly/type-checker-emit-after-unreachable.s

+# We had a regression where an else instruction would be omitted if it followed
+# return (i.e. it was in an unreachable state).
+# See: https://github.com/llvm/llvm-project/issues/56935
+
+# RUN: llvm-mc -triple=wasm32-unknown-unknown < %s | FileCheck %s
+
+foo:
+  .functype foo () -> (i32)
+  i32.const 1
+  if i32
+    i32.const 2
+    return
+  else
+    i32.const 3
+  end_if
+  end_function
+
+# CHECK-LABEL: foo:
+# CHEKC-NEXT: .functype foo () -> (i32)
+# CHEKC-NEXT: i32.const 1
+# CHEKC-NEXT: if i32
+# CHEKC-NEXT: i32.const 2
+# CHEKC-NEXT: return
+# CHEKC-NEXT: else
+# CHEKC-NEXT: i32.const 3
+# CHEKC-NEXT: end_if
+# CHEKC-NEXT: end_function