compiler: allow to use conditional returns in inlined functions

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/compiler/testdata/inline/c/null.go

@@ -0,0 +1,22 @@
+package c
+
+func Is42(a int) bool {
+	if a == 42 {
+		return true
+	}
+	return false
+}
+
+func MulIfSmall(n int) int {
+	if n < 10 {
+		return n * 2
+	}
+	return n
+}
+
+func Transform(a, b int) int {
+	if Is42(a) && !Is42(b) {
+		return MulIfSmall(b)
+	}
+	return MulIfSmall(a)
+}