A workaround in codeview debuginfo generation was to declare a condfail
instruction as inlined to avoid using 0 as an artificial line location.
However, this was done without accounting for scopes that were already
legitimately inlined. So we'd end up with a condfail inlined from
function B into function A and then the IRGen for the condfail was
being given a debugloc claiming to have been inlined again. This was
causing a sitaution where we'd have debug info forfunctionA owning an
instruction which claimed it was owned by a different function.
Fix this by first checking if the `LastScope` was already inlined and,
if so, just used that scope instead.
