Scalar multiply expects a 'd' register.  The "w" (float) uses 's' for float
and wont work with the multiply in 32 bit (it does in 64 bit).
A vector 2 of float passes as 'd' register.
A vector 4 of float passes as 'q' register.
This change copies the float into the first entry of a vector 2
and passes that.  The optimizer removes the extra copy, allowing
the single float to use referenced as

Test: LibYUVPlanarTest.TestByteToFloat
Bug: libyuv:786
Change-Id: I8773c5bae043c7b84e1d1db7fdea6731aa0b1323
Reviewed-on: https://chromium-review.googlesource.com/973984
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>