[bigint,compiler] Support bigints in typeof operator.

This adds a bigint branch to the typed-optimization of the TypeOf
bytecode. The implementation of the TestTypeOf bytecode already supports
bigints, as does the Typeof stub.

R=jarin@chromium.org

Bug: v8:6791
Change-Id: Ib9a21f3fc48d57873b014a01c68a143bfb8ac6c6
Reviewed-on: https://chromium-review.googlesource.com/778880
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49509}

src/compiler/typed-optimization.cc

@@ -465,6 +465,8 @@ Reduction TypedOptimization::ReduceTypeOf(Node* node) {
     return Replace(jsgraph()->Constant(f->number_string()));
   } else if (type->Is(Type::String())) {
     return Replace(jsgraph()->Constant(f->string_string()));
+  } else if (type->Is(Type::BigInt())) {
+    return Replace(jsgraph()->Constant(f->bigint_string()));
   } else if (type->Is(Type::Symbol())) {
     return Replace(jsgraph()->Constant(f->symbol_string()));
   } else if (type->Is(Type::OtherUndetectableOrUndefined())) {