Avoid return blocks that have only a single predecessor

Currently, we always create a dedicated "return" basic block, but when
there's only a single predecessor for that block, it can be merged with
that predecessor. We can achieve that merge by only creating the return
block on demand, avoiding its creation when its not required.

Reduces the pre-optimization size of librustc.ll created with --passes ""
by about 90k lines which equals about 4%.

0 files changed, 0 insertions, 0 deletions