rustc: move the check_loop pass earlier.

This pass is purely AST based, and by running it earlier we emit more
useful error messages, e.g. type inference fails in the case of `let r =
break;` with few constraints on `r`, but its more useful to be told that
the `break` is outside a loop (rather than a type error) when it is.

Closes #13292.
auto merge of #13330 : huonw/rust/loop-error, r=alexcrichton

rustc: move the check_loop pass earlier.

This pass is purely AST based, and by running it earlier we emit more
useful error messages, e.g. type inference fails in the case of 
`let r = break;` with few constraints on `r`, but it's more useful to be told that
the `break` is outside the loop (rather than a type error) when it is.

Closes #13292.