The number of feedback vector slots is currently stored in the
{WasmFunction}, returned in the {WasmCompilationResult}, and implicitly
stored as the size of the {call_targets} vector in
{FunctionTypeFeedback}.

This CL uses the latter as the source of truth, encapsulated in a new
{NumFeedbackSlots} function. This can be updated when adding new kinds
of feedback that need additional slots.
For now, the implementation of {NumFeedbackSlots} requires taking a
mutex, which we can hopefully avoid when productionizing speculative
inlining. We also take the mutex on every Liftoff compilation, which
adds synchronization between concurrent compilation which we previously
tried very hard to avoid (because it introduced significant overhead for
eager compilation).

As a nice side-effect, this CL reduces the per-function overhead by 8
bytes, independent of enabled features.

R=jkummerow@chromium.org

Bug: v8:13209
Change-Id: I2fe5f7fe73154328032a3f0961e88d068c5d07ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899299Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83253}