nir/opcodes: Simplify the expressions for [un]pack_double

nir/opcodes: Simplify the expressions for [un]pack_double

The new expressions are more explicit in terms of where the bits go so it's
a little easier to tell what's going on.  This is the way GLSL specifies
things so it's a bit easier to verify too.  It also has the benifit that
the new expressions easily vectorize so we can constant-fold vector forms
of the _split versions correctly.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>