On ia32 return statements in C++ automatically convert signalling NaNs
to quiet NaNs, even when bit_cast is used. This CL removes all uses of
bit_cast<float> and bit_cast<double> in the wasm compiler and wasm
interpreter.

R=titzer@chromium.org, clemensh@chromium.org

Review-Url: https://codereview.chromium.org/2639353002
Cr-Commit-Position: refs/heads/master@{#42512}