LoopUnrollAndJam] Only allow loops with single exit(ing) blocks

Summary:
This resolves an issue posted on Bugzilla. https://bugs.llvm.org/show_bug.cgi?id=48764
In this issue, the loop had  multiple exit blocks, which resulted in the
function getExitBlock to return a nullptr, which resulted in hitting the assert.
This patch ensures that loops which only have one exit block as allowed to be
unrolled and jammed.

Reviewed By: Whitney, Meinersbur, dmgreen

Differential Revision: https://reviews.llvm.org/D95806