The dynamic tiering budget slot will never be used if dynamic tiering is
disabled. As it's un untagged field (never visited by GC), we can just
leave it uninitialized.

Similarly, the feedback vector slot is only used (and visited by the GC)
if --wasm-speculative-inlining is enabled.

Since both is disabled by default, we can save two spills in each
function, saving 16 bytes on x64.

Drive-by: Add code comments for both code blocks.

R=jkummerow@chromium.org

Change-Id: If325e795f6368e02ed687697c4bdac208214103b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3468348Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79192}