Support bitcasts between scalars and short vectors.

Support bitcasts between scalars and short vectors.

Subzero only supports 128-bit vectors currently, but we need to support
bitcasting between scalars and (emulated) short vectors of the same
size. Subzero implicitly supports it on x86 by using movd instructions,
but on ARM we have to emulate it in Reactor until support for it is
added in Subzero.

Bug swiftshader:48

Change-Id: I95accbc3665815a46dac8e52ce30939ac64aaf44
Reviewed-on: https://swiftshader-review.googlesource.com/10929
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>