[vm/switchable_calls] Clean up assert in monomorphic miss jit handler.
[vm/switchable_calls] Clean up assert in monomorphic miss jit handler.

The check for number of isolates was racy because isolates might be exiting, number of isolates in the group going down as we enter ic cache miss runtime handler.
This caused flaky failures on debug buildbots.

TEST=ci
Fixes https://github.com/dart-lang/sdk/issues/47132

Change-Id: Ie78a80fecb30d2cc40635acd465078cb0f22a234
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212487
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk