Float16ToFloat32 is now required by constant folding of
float pack/unpack built-ins as well, moving it to commom
so that it is available in libangle_common instead of libANGLE.

BUG=angleproject:913
TEST=angle_unittests

Change-Id: I1e23bb98c1b725c79f933a66ddb9986133fec19c
Reviewed-on: https://chromium-review.googlesource.com/282961Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>