Since there usually aren't many pending 32-bit constants, we can avoid
the cost of malloc/free for most compiles by using a base::SmallVector
instead of std::vector.

I picked a value of 32 entries as the SmallVector's inline size, based
on compiling three.js and jquery with Sparkplug, and printing the size
of the constant pool when emitted. 93% of emitted constant pools had
<=32 entries.

Bug: v8:11420
Change-Id: I7d62da74c60feae08f8a4b16b1e7f93ea69d2c95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2840447Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74092}