Move dead_code related tests to test/ui/dead-code

This helps organize the tests better.  I also renamed several of the tests to remove redundant dead-code in the path, and better match what they're testing

src/test/ui/dead-code-impl.rs

@@ -1,17 +0,0 @@
-// run-pass
-
-#![deny(dead_code)]
-
-pub struct GenericFoo<T>(T);
-
-type Foo = GenericFoo<u32>;
-
-impl Foo {
-    fn bar(self) -> u8 {
-        0
-    }
-}
-
-fn main() {
-    println!("{}", GenericFoo(0).bar());
-}