Restore the basic write barrier elimination that we used to run as part
of the simplified lowering phase (in ChangeLowering actually) before, by
moving the write barrier computation to SimplifiedLowering where we can
still look at types and consider the heap/isolate, and just update the
WriteBarrierKind in the FieldAccess/ElementAccess that we later use when
lowering to a machine Load/Store.

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux64_tsan_rel
R=mstarzinger@chromium.org
BUG=v8:4969,chromium:608636
LOG=n

Review-Url: https://codereview.chromium.org/1938993002
Cr-Commit-Position: refs/heads/master@{#35969}