During iterate_solution(), live-through values may be converted to
solver variables so they can be moved out of the way in order to satisfy
all constraints. Make sure that the instruction's operand constraints
are also considered for these new variables.
Add a program_complete_input_constraints() which turns all the
instruction's input operands into variables with the proper constraints.
That makes it safe for try_add_var() to re-add these values as variables
with looser generic constraints.
The solver's add_var() function is split into three functions: add_var
for use before inputs_done(), and add_killed_var/add_through_var for use
after.
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