Re-enable "[SCEV] Make computeExitLimit more simple and more powerful"

This patch was temporarily reverted because it has exposed bug 37229 on
PowerPC platform. The bug is unrelated to the patch and was just a general
bug in the optimization done for PowerPC platform only. The bug was fixed
by the patch rL331410.

This patch returns the disabled commit since the bug was fixed.

llvm-svn: 331427