Fixes for bugs around unreachable code

Remove the ICE check for expression behaviors always being either `{Next}` or `{Next, Discard}`. Unreachable code may be result in something else.

Add the RemoveUnreachableStatements transform to the SPIR-V writer sanitizer transform list. The writer cannot correctly handle unreachable statements.

Bug: tint:1369
Bug: chromium:1285622
Change-Id: I9fa54c6d2096b1ee633dd551b628c7dd3ba64fb5
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/76300
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/resolver/resolver.cc

@@ -1149,18 +1149,6 @@ sem::Expression* Resolver::Expression(const ast::Expression* root) {
       return nullptr;
     }
 
-    // https://www.w3.org/TR/WGSL/#behaviors-rules
-    // an expression behavior is always either {Next} or {Next, Discard}
-    if (sem_expr->Behaviors() != sem::Behavior::kNext &&
-        sem_expr->Behaviors() != sem::Behaviors{sem::Behavior::kNext,  // NOLINT
-                                                sem::Behavior::kDiscard} &&
-        !IsCallStatement(expr)) {
-      TINT_ICE(Resolver, diagnostics_)
-          << expr->TypeInfo().name
-          << " behaviors are: " << sem_expr->Behaviors();
-      return nullptr;
-    }
-
     builder_->Sem().Add(expr, sem_expr);
     if (expr == root) {
       return sem_expr;