We had IsJSHandlerOnTop and IsExternalHandlerOnTop, which were almost
opposites but not quite. We often did the same computation repeatedly
for determining which kind of a handler is at the top (if any).

This CL simplifies the logic, and only does the three-way logic once:
either there's an external handler, a JS handler, or neither.

It also removes dead code from Isolate::ReportPendingExceptions: we
already do an early return if there's a JS handler on top, so we don't
need to re-check.

Bug: v8:12437
Change-Id: Ic15675bf2177772037d9fcec31c79019e4f0e02c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3302802Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78129}