The scheduled_nodes_ vector is used to maintain a per-block list of
non-fixed nodes. For most blocks this list remains empty, so lazily
initialize it instead of pre-allocating to save memory.

Also pre-reserve an extra 10% of blocks to avoid reallocting space in the
vector when fusing floating control creates new basic blocks.

BUG=chromium:700364

Change-Id: I9876e6a42bc90c9bff5838620365c18609ed1ee9
Reviewed-on: https://chromium-review.googlesource.com/458919Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44152}