[ARM] Add low overhead loops terminators to AnalyzeBranch

This treats low overhead loop branches the same as jump tables and
indirect branches in analyzeBranch - they cannot be analyzed but the
direct branches on the end of the block may be removed. This helps
remove the unnecessary branches earlier, which can help produce better
codegen (and change block layout in a number of cases).

Differential Revision: https://reviews.llvm.org/D94392