Before emitting the safepoint table, remove consecutive identical
entries (idential except for the pc of course). The lookup then
searches for the last entry whose pc is <= the wanted pc.
The lookup procedure can still be optimized to use binary search
laster.

This change decreases code size for wasm by 27.6% (on the unity
benchmark).

BUG=v8:6434

Change-Id: I03481721fe666cd2c50a383380c74b06edf39106
Reviewed-on: https://chromium-review.googlesource.com/512542
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45491}