{WhichPowerOf2} is basically the same as {CountTrailingZeros}, with a
restriction to powers of two. Since it does not use or depend on any v8
internals, it can be moved to src/base/bits.h.
This CL also changes the implementation to use the CTZ builtin if
available, and falls back to popcnt otherwise.

Drive-by: Make it constexpr, and rename to {WhichPowerOfTwo}.

R=sigurds@chromium.org

Bug: v8:9810, v8:8912
Change-Id: I8368d098f9ab1247f3b9f036f1385a38de10cc6a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1903966Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64851}