The basic block instrumentation currently uses 32-bit integers, which
could overflow during a long profiling session. I considered upgrading
them to 64-bit integers, but generating the correct instrumentation code
for various architectures would be rather non-trivial. Instead, this
change uses 64-bit floating-point values, which are simple and also have
the nice behavior that they saturate rather than overflowing.

Bug: v8:10470
Change-Id: I60f7456cb750091809803c03a85dd348dc614b58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2545573Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#71297}