[CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks
This patch has trigger a bug of r368339, and the r368339 has been reverted, So upstream this patch again.
In block-placement pass, it will create some patterns for unconditional we can do the simple early retrun.
But the early-ret pass is before block-placement, we don't want to run it again.
This patch is to do the simple early return to optimize the blocks at the last of block-placement.
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D63972