Since floating-point scalars are stored in vector registers, Subzero
allows us to just bitcast between them, eliminating a load and insert
before shuffling.

Change-Id: Ibccf242fd4cfc28604f35f420a04fd4ee6eabe52
Reviewed-on: https://swiftshader-review.googlesource.com/8575Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>