Previously, the handle's location was used as a proxy for the heap
object, i.e, we put the handle into the constant pool, to avoid the
need for GC visiting the constant pool entries during code generation.
The handle locations are replaced by the corresponding heap object
when the code is copied to the heap.

This CL changes the handling in the assembler: Instead of putting
in the handle location (which is a machine word) we put in a small
index number into a table. This will be useful for putting 32bit
constants into the constant pool.

This new approach also has the advantage that ordering the
constant pool entries by value produces a deterministic order
after this change.

Change-Id: Id47d56d487a0b64d1d1504a47937c8779ee02b13
Bug: v8:7703
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1648094
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62144}