Revert "Distinguish beween final incremental mark-compact and full mark-compact event in IdleNotification."

This reverts commit d15d453fa5aaeeb1f7e010a1baf71e7b3306802b for
breaking cctest/test-api/Threading3.

TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/758163003

Cr-Commit-Position: refs/heads/master@{#25506}
Document that Isolate::GetCurrent() must not be called before initialization

Also, add a check for debug mode.

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/758043002

Cr-Commit-Position: refs/heads/master@{#25507}
[turbofan] Use special constant type for RPO block number operands.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/758073002

Cr-Commit-Position: refs/heads/master@{#25508}