There are wasm operations which operate on floats or double, but they
need to preserve the exact bit pattern. Thus they cannot be stored and
passed as float or double, since that might flip the signaling NaN bit.
This CL extends WasmValue to store floats and doubles as bit pattern,
and adds accessors to extract them as Float32 or Float64.
The interpreter is changed to execute certain operations (i32.abs,
i32.neg, i64.abs, i64.neg, f32.reinterpret/i32, f64.reinterpret/i64) on
boxed floats.

R=titzer@chromium.org

Bug: v8:6954
Change-Id: I0251d1a67b6caf593194d4eb292a325cdd3f20cf
Reviewed-on: https://chromium-review.googlesource.com/730716
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48819}