On ia32/x64 when casting a sNan to double, the signalling
bit is flipped to qNan.

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2660873003
Cr-Commit-Position: refs/heads/master@{#42786}