'Assertion !cx.terminated failed' involving nested rets

[jruderman]

fn f() -> int { ret (ret 2) + 3; }
fn main() { }

Results in an assertion in a function called "Br":

upcall fail 'Assertion !cx.terminated failed', ../src/comp/middle/trans_build.rs:34

Same error message as in #893, but a very different testcase.

[marijnh]

Fixing this will involve either inserting checks with early-return paths after each and every call to trans_expr and trans_lval, or adding some machinery to the builder to simply ignore extra code added to terminated blocks.

[msullivan]

This looks a lot like all of other bottom returning !cx.terminated bugs that tjc was fixing. Is it different in some way?