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-Original-Commit-Position: refs/heads/master@{#42512}
Committed: https://chromium.googlesource.com/v8/v8/+/7739affa5b57e0d28674d476f63de60d71728fb6
Review-Url: https://codereview.chromium.org/2639353002
Cr-Commit-Position: refs/heads/master@{#42545}