improvements to folly::doNotOptimizeAway

improvements to folly::doNotOptimizeAway

Summary:
A common failure mode for folly::doNotOptimizeAway is to pass it a
pointer to a complex object that the caller wants to ensure is fully
calculated, which doesn't actually ensure that.  Also, the GCC and
MSVC implementations used inline assembly with an in-out parameter,
which causes a store.  A survey of many usages of doNotOptimizeAway
found only 1 that relies on this make-unpredictable behavior.

This diff makes doNotOptimizeAway(&x) equivalent to doNotOptimizeAway(x)
for GCC and clang, and makes it a read-only sink.  For the rare
case that the benchmark wants to disable subexpression elimination,
constant propagation, or power reduction, there is a new function
makeUnpredictable.  It also merges the clang and GCC implementations,
removing a potential bias in our microbenchmarks.

Reviewed By: davidtgoldblatt

Differential Revision: D4074670

fbshipit-source-id: 43f02e7fe149147bb172babe77787dea06e098fa