pass,printer: display required ISA features (#120)

Fixes #119

pass/isa_test.go

@@ -0,0 +1,25 @@
+package pass
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/mmcloughlin/avo/ir"
+)
+
+func TestRequiredISAExtensions(t *testing.T) {
+	f := ir.NewFunction("ISAs")
+	f.AddInstruction(&ir.Instruction{ISA: nil})
+	f.AddInstruction(&ir.Instruction{ISA: []string{"B", "A"}})
+	f.AddInstruction(&ir.Instruction{ISA: []string{"A", "C"}})
+
+	err := RequiredISAExtensions(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expect := []string{"A", "B", "C"}
+	if !reflect.DeepEqual(f.ISA, expect) {
+		t.FailNow()
+	}
+}