AddImmediate ends up pushing repeated immediates very often
unecessarily. Add support for Int64 immediates being inlined into
InstructionOperand if they fit into the payload (which is almost always
the case). Also add a seperate rpo_immediate vector for RPO numbers to
avoid having to add them to the immediates_ vector multiple times.
Ideally the RPO values would also be inlined, however JumpThreading
needs to patch RPO targets throughout the instruction stream, so we
need an indirection.

Change-Id: I75b1cdb05917f85d4f740a34c3720dd9cf0ee29c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2782282
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73687}