internal/load: support additional MOVQ forms

The Go assembler merges MOVD/MOVQ instruction forms. The logic in the
avo instruction loader was discarding the MOVD forms. This diff should
merge them correctly.

Updates #50

tests/fixedbugs/issue50/issue50_test.go

@@ -0,0 +1,15 @@
+package issue50
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out issue50.s -stubs stub.go
+
+func TestIssue50(t *testing.T) {
+	expect := func(x uint32) uint32 { return x }
+	if err := quick.CheckEqual(Issue50, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}