If multiple isolates share the same module, and the debugger gets
enabled, then we trigger tier down in each isolate separately. To avoid
generating too much code, we only recompile functions that are not
already in the right tier.

This CL is only the first step towards an actual fix. Since we only
check already installed code (and ignore compilations that are already
scheduled), we might still compile the same functions multiple times. A
second CL will make sure that only one recompilation is running at the
same time.

R=thibaudm@chromium.org

Bug: chromium:1084369, v8:10359
Change-Id: Ic4f9afac1add0fe8ad9e5d68f22d3d41ba2e52be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2213438Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67957}