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>