nir: Add a fdot instruction that replicates the result to a vec4

nir: Add a fdot instruction that replicates the result to a vec4

Fortunately, nir_constant_expr already auto-splats if "dst" never shows up
in the constant expression field so we don't need to do anything there.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>