nir/from_ssa: Don't lower constant SSA values to registers

nir/from_ssa: Don't lower constant SSA values to registers

Backends want to be able to do special things with constant values such as
put them into immediates or make decisions based on whether or not a value
is constant.  Before, constants always got lowered to a load_const into a
register and then a register use.  Now we leave constants as SSA values so
backends can special-case them if they want.  Since handling constant SSA
values is trivial, this shouldn't be a problem for backends.

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