In 32-bit mode, all function arguments are passed on the stack, not in
registers.
This ABI support is not complete or properly tested, but at least it
doesn't try to pass arguments in r8.
Fixes#147.
The Solver::reassign_in() method would previously not record fixed
register assignments for values that are already in the correct
register. The register would simply be marked as unavailable for the
solver.
This did have the effect of tripping up the sanity checks in
Solver::add_var() when that method was called with such a "reassigned"
value. The function can be called for a value that already has a fixed
assignment, but the sanity checks want to make sure the variable
constraints are compatible with the existing fixed assignment. When no
such assignment could be found, the method panicked.
To fix this, make sure that even identity reassignments are recorded
in the assignments vector. Instead, filter the identity assignments out
before scheduling a move sequence for the assignments.
Also add some debug tracing to the regalloc solver.
