[SimpleLoopUnswitch] Form dedicated exits after trivial unswitches.

Summary:
Form dedicated exits after trivial unswitches.
Fixes PR38737, PR38283.

Reviewers: chandlerc, fedor.sergeev

Subscribers: sanjoy, jlebar, uabelho, llvm-commits

Differential Revision: https://reviews.llvm.org/D51375

llvm-svn: 340871