checker: add a missing return regression test for if branches too

vlib/v/checker/tests/return_missing_if_else_simple.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/return_missing_if_else_simple.vv:1:1: error: missing return at end of function `if_no_returns`
+    1 | fn if_no_returns() int {
+      | ~~~~~~~~~~~~~~~~~~~~~~
+    2 |     if true {
+    3 |         println('no return in then')
+vlib/v/checker/tests/return_missing_if_else_simple.vv:10:1: error: missing return at end of function `if_no_return_in_else`
+    8 | }
+    9 | 
+   10 | fn if_no_return_in_else() int {
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   11 |     if true {
+   12 |         return 123

vlib/v/checker/tests/return_missing_if_else_simple.vv

@@ -0,0 +1,35 @@
+fn if_no_returns() int {
+	if true {
+		println('no return in then')
+	} else {
+		println('no return in else')
+	}
+	println('hello')
+}
+
+fn if_no_return_in_else() int {
+	if true {
+		return 123
+	} else {
+		println('no return in else')
+	}
+	println('hello')
+}
+
+fn if_returns_in_both_branches() int {
+	// The if here always returns, so the function
+	// returns too:
+	if true {
+		return 123
+	} else {
+		return 456
+	}
+	// TODO: should produce a warning/error about a statement after a return.
+	println('hello')
+}
+
+fn main() {
+	println(if_no_returns())
+	println(if_no_return_in_else())
+	println(if_returns_in_both_branches())
+}