rustc: Don't generate code for unreachable expressions

The way we deal with unreachable expressions in trans is pretty ad hoc,
but this at least doesn't make it worse, and eliminates the LLVM
assertion failure reported in #5741.
auto merge of #6665 : catamorphism/rust/issue-5741, r=nikomatsakis

r? @nikomatsakis The way we deal with unreachable expressions in trans is pretty ad hoc,
but this at least doesn't make it worse, and eliminates the LLVM
assertion failure reported in #5741.