Remove checks for IC hotness from Ignition tiering up decision since this is
not relevent for full-codegen compilation. Also make the decision about what
tier we are moving to more explicit and visible in --trace-opt.

BUG=v8:4280
LOG=N

Review-Url: https://codereview.chromium.org/1969773002
Cr-Commit-Position: refs/heads/master@{#36260}