Merge the tail block into the loop in cases where the main loop body
exits early, subject to profitability constraints. This will coalesce
the loop body into fewer blocks.
For example:
loop: loop:
// loop body // loop body
if (...) jump exit --> // more body
more: if (...) jump exit
// more body jump loop
jump loop
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297033 91177308-0d34-0410-b5e6-96231b3b80d8
