They did not preserve the bit pattern of nans before. Now they do.
Also, add some tests for these instructions.

R=ahaas@chromium.org, rodolph.perfetta@arm.com

Bug: v8:6947
Change-Id: I189720cd47e1768194567a41371fc9586b414c45
Reviewed-on: https://chromium-review.googlesource.com/722979
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Rodolph Perfetta <rodolph.perfetta@arm.com>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48672}