Fixed incorrect register lifetime in for loops

tests/programs/fibonacci.q

@@ -0,0 +1,26 @@
+main() {
+	for i := 0..10 {
+		assert fib1(i) == fib2(i)
+	}
+}
+
+fib1(n int) -> int {
+	b := 0
+	c := 1
+
+	for 0..n {
+		a := b
+		b = c
+		c = a + b
+	}
+
+	return b
+}
+
+fib2(n int) -> int {
+	if n <= 1 {
+		return n
+	}
+
+	return fib2(n - 1) + fib2(n - 2)
+}