Drop tracking: improve break and continue handling

This commit fixes two issues.

One, sometimes break or continue have a block target instead of an
expression target. This seems to mainly happen with try blocks. Since
the drop tracking analysis only works on expressions, if we see a block
target for break or continue, we substitute the last expression of the
block as the target instead.

Two, break and continue were incorrectly being treated as the same, so
continue would also show up as an exit from the loop or block. This
patch corrects the way continue is handled by keeping a stack of loop
entry points and uses those to find the target of the continue.

compiler/rustc_ast/src/ast.rs

@@ -54,7 +54,7 @@ mod tests;
 /// ```
 ///
 /// `'outer` is a label.
-#[derive(Clone, Encodable, Decodable, Copy, HashStable_Generic)]
+#[derive(Clone, Encodable, Decodable, Copy, HashStable_Generic, Eq, PartialEq)]
 pub struct Label {
     pub ident: Ident,
 }