DropRegisterAtEnd is used to free a register, to make space for the
result of a node. Normally this frees up an input that is dead at the
end of the node's lifetime, but under high local variable pressure, we
might not have a dead value to drop.

In these cases we have to spill a register through the normal spilling
mechanism. Additionally, allow freeing up a blocked free register (i.e.
a temporary) if this is possible.

Bug: v8:7700
Change-Id: I0099751918cf5cb65c2a09337a3f080eb2c4dd14
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3769833Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81804}