This corrects the case when we need to allocate a
blocked register, but the blockage happens after a
use as an instruction input, and there's no place to
split before that.

BUG=v8:5888

Review-Url: https://codereview.chromium.org/2652153005
Cr-Original-Commit-Position: refs/heads/master@{#42706}
Committed: https://chromium.googlesource.com/v8/v8/+/ca779b29a60785d962d7cd636f6b0f09e66130b3
Review-Url: https://codereview.chromium.org/2652153005
Cr-Commit-Position: refs/heads/master@{#42710}