Fix a couple of wrong-code bugs in switch-on-constant optimization:

 * recurse through intermediate LabelStmts and AttributedStmts when checking
   whether a statement inside a switch declares a variable
 * if the end of a compound statement is reachable from the chosen case label,
   and the compound statement contains a variable declaration, it's not valid
   to just emit the contents of the compound statement -- we must emit the
   statement itself or we lose the scope (and thus end lifetimes at the wrong
   point)

llvm-svn: 281797