Rollup merge of #22527 - dotdash:if-loop, r=huonw

 In `if loop {} {}`, the `if` is actually unreachable, but we didn't
handle that correctly and when trying to translate the `if` we tried to
branch on the \"return value\" of the loop expression, which is not an
`i1` and therefore triggered an LLVM assertion.

diff --cc src/librustc_trans/trans/controlflow.rs
Simple merge