check if the type of a local variable contains illegal region

also, try to suppress some of the duplicate errors---in general
more work is needed to make these error messages comprehensible
to ordinary humans

src/test/compile-fail/regions-escape-loop-via-variable.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let x = 3;
+
+    // Here, the variable `p` gets inferred to a type with a lifetime
+    // of the loop body.  The regionck then determines that this type
+    // is invalid.
+    let mut p = //! ERROR reference is not valid
+        &x;
+
+    loop {
+        let x = 1 + *p;
+        p = &x;
+    }
+}