[CSA] Fix the typed verison of Select to strictly check types.

Previously, typing errors would be hidden by the SloppyTNode cast. Both
lambdas should always return strictly checked types that match the
templated type of the select, otherwise unchecked values can reach
TNodes.

Bug: v8:7479
Change-Id: Icd316c21a30c8c8305b7740c985efaa54c9a7015
Reviewed-on: https://chromium-review.googlesource.com/934455Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51515}

src/code-stub-assembler.h

@@ -356,15 +356,11 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
   template <class A, class F, class G>
   TNode<A> Select(SloppyTNode<BoolT> condition, const F& true_body,
                   const G& false_body, MachineRepresentation rep) {
-    return UncheckedCast<A>(
-        Select(condition,
-               [&]() -> Node* {
-                 return base::implicit_cast<SloppyTNode<A>>(true_body());
-               },
-               [&]() -> Node* {
-                 return base::implicit_cast<SloppyTNode<A>>(false_body());
-               },
-               rep));
+    return UncheckedCast<A>(Select(
+        condition,
+        [&]() -> Node* { return base::implicit_cast<TNode<A>>(true_body()); },
+        [&]() -> Node* { return base::implicit_cast<TNode<A>>(false_body()); },
+        rep));
   }
 
   Node* SelectConstant(Node* condition, Node* true_value, Node* false_value,