nir: Make load_const SSA-only

nir: Make load_const SSA-only

As it was, we weren't ever using load_const in a non-SSA way.  This allows
us to substantially simplify the load_const instruction.  If we ever need a
non-SSA constant load, we can do a load_const and an imov.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>