Optimize code placement in loop to eliminate unconditional branches or move unconditi...

Optimize code placement in loop to eliminate unconditional branches or move unconditional branch to the outside of the loop. e.g.

///       A:
///       ...
///       <fallthrough to B>
///
///       B:  --> loop header
///       ...
///       jcc <cond> C, [exit]
///
///       C:
///       ...
///       jmp B
///
/// ==>
///
///       A:
///       ...
///       jmp B
///
///       C:  --> new loop header
///       ...
///       <fallthough to B>
///
///       B:
///       ...
///       jcc <cond> C, [exit]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71209 91177308-0d34-0410-b5e6-96231b3b80d8