has_slot_use is computed early, and we need it to determine if we need
to generate SpillRanges. After splintering, however, the information may
be incorrect - e.g. just the splinter may have slot uses, and not the
original.

BUG=

Review-Url: https://codereview.chromium.org/2312523002
Cr-Commit-Position: refs/heads/master@{#39147}