[CodeGen] Check for HardwareLoop Latch ExitBlock

[CodeGen] Check for HardwareLoop Latch ExitBlock

The HardwareLoops pass finds exit blocks with a scevable exit count.
If the target specifies to update the loop counter in a register,
through a phi, we need to ensure that the exit block is a latch so
that we can insert the phi with the correct value for the incoming
edge.

Differential Revision: https://reviews.llvm.org/D63336

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363556 91177308-0d34-0410-b5e6-96231b3b80d8