Always call reassign_in for register ABI arguments.

Even if an argument is already in the correct register, make sure that
we detect conflicts by registering the no-op move. This also means that
the ABI argument register won't be turned into a variable for the
solver.

lib/cretonne/src/regalloc/coloring.rs

@@ -430,14 +430,13 @@ impl<'a> Context<'a> {
                          locations: &EntityMap<Value, ValueLoc>) {
         for (abi, &value) in abi_types.iter().zip(dfg.inst_variable_args(inst)) {
             if let ArgumentLoc::Reg(reg) = abi.location {
-                let cur_reg = self.divert.reg(value, locations);
-                if reg != cur_reg {
                 if let Affinity::Reg(rci) =
                     self.liveness
                         .get(value)
                         .expect("ABI register must have live range")
                         .affinity {
                     let rc = self.reginfo.rc(rci);
+                    let cur_reg = self.divert.reg(value, locations);
                     self.solver.reassign_in(value, rc, cur_reg, reg);
                 } else {
                     panic!("ABI argument {} should be in a register", value);
@@ -445,7 +444,6 @@ impl<'a> Context<'a> {
             }
         }
     }
-    }
 
     // Find existing live values that conflict with the fixed input register constraints programmed
     // into the constraint solver. Convert them to solver variables so they can be diverted.

lib/cretonne/src/regalloc/solver.rs

@@ -364,6 +364,7 @@ impl Solver {
         }
         self.regs_in.free(rc, from);
         self.regs_out.take(rc, to);
+        if from != to {
             self.assignments
                 .insert(Assignment {
                             value,
@@ -372,6 +373,7 @@ impl Solver {
                             to,
                         });
         }
+    }
 
     /// Add a variable representing an input side value with an existing register assignment.
     ///