AArch64: Add missing branch relaxation tests

The branch relaxation pass has the worst test coverage
of any pass in AArch64. Add a few tests that hit some
large pieces of code in the pass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277428 91177308-0d34-0410-b5e6-96231b3b80d8