Adds support for tracking the instruction range of spilled operands,
and then allocating spill slots to these ranges. It also adds some
unittests covering spill slot allocation.

Spill slots are allocated in a linear fashion, running through the
instruction stream in a linear order, ensuring that no spill operand
is allocated to a same spill slot that is already assigned to during
this whole start / end range. This isn’t optimal, since it doesn’t
take into account holes in these ranges (e.g, blocks between start
and end that aren’t dominated by the start), but in practice rarely
leads to more than one extra spill slot being allocated compared to
the current allocator.

BUG=v8:9684

Change-Id: Iedee7bcf552080e5b4b6a2f4e96b78b6c1396cab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2297470Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69107}