Adds a pending operand type for use with the fast register allocator.
These operands chain together multiple operands together, enabling
the allocator to keep track of multiple pending operands, then
replace them all with the allocated operand in one go.

BUG=v8:9684

Change-Id: I5d8150f3f26549a747a2e89e32e31135e89dff9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2292302
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69019}