[turbofan] Be more verbose when failing monotonicity check.

R=jarin@chromium.org

Bug: chromium:906567
Change-Id: I280569e09529457814ebc1cb8f299a80aa40a6c4
Reviewed-on: https://chromium-review.googlesource.com/c/1345992Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57676}

src/compiler/typer.cc

@@ -328,7 +328,14 @@ class Typer::Visitor : public Reducer {
         current = Weaken(node, current, previous);
       }
 
-      CHECK(previous.Is(current));
+      if (V8_UNLIKELY(!previous.Is(current))) {
+        std::ostringstream ostream;
+        previous.PrintTo(ostream);
+        ostream << " -> ";
+        current.PrintTo(ostream);
+        FATAL("UpdateType error for operator %s:\n%s\n",
+              IrOpcode::Mnemonic(node->opcode()), ostream.str().c_str());
+      }
 
       NodeProperties::SetType(node, current);
       if (!current.Is(previous)) {