test: Add a test for mismatched types of if branches

Author: brson

src/test/compile-fail/if-branch-types.rs

@@ -0,0 +1,9 @@
+// error-pattern:mismatched types
+
+fn main() {
+  auto x = if (true) {
+    10
+  } else {
+    10u
+  };
+}