When merging blocks in the mid-tier register allocator, try to avoid
spilling registers if we can instead introduce a gap move to move the
register choosen to hold a virtual register to the register that is
holding that virtual register in the successor block. This gives
around 1.5% speedup on Octane.

BUG=v8:9684

Change-Id: I352d1e0e2c3b89ab6bd203d76339b93ef474028f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2466117
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70534}