Add a FixedTied constraint kind for operand constraints.

Fixes #175.

The Intel division instructions have fixed input operands that are
clobbered by fixed output operands, so the value passed as an input will
be clobbered just like a tied operand.

The FixedTied operand constraint is used to indicate a fixed input
operand that has a corresponding output operand with the same fixed
register.

Teach the spiller to teach a FixedTied operand the same as a Tied
operand constraint and make sure that the input value is killed by the
instruction.

lib/cretonne/src/regalloc/spilling.rs

@@ -301,6 +301,10 @@ impl<'a> Context<'a> {
                     // A tied operand must kill the used value.
                     reguse.tied = !lr.killed_at(inst, ebb, &self.cur.func.layout);
                 }
+                ConstraintKind::FixedTied(_) => {
+                    reguse.fixed = true;
+                    reguse.tied = !lr.killed_at(inst, ebb, &self.cur.func.layout);
+                }
                 ConstraintKind::Reg => {}
             }
             if lr.affinity.is_stack() {