[AArch64] Move the branch relaxation pass after BTI insertion

Summary:
Inserting BTI instructions can push branch destinations out of range.

The branch relaxation pass itself cannot insert indirect branches since `TargetInstrInfo::insertIndirecrtBranch` is not implemented for AArch64 (guess +/-128 MB direct branch range is more than enough in practice).

Testing this is a bit tricky.

The original test case we have is 155kloc/6.1M. I've generated a test case using this program:
```

int main() {
  std::cout << R"src(int test();
void g0(), g1(), g2(), g3(), g4(), e();

void f(int v) {
  if ((test() & 2) == 0) {
  switch (v) {
  case 0:
    g0();
  case 1:
    g1();
  case 2:
    g2();
  case 3:
    g3();
  }
)src";

  const int N = 8176;

  for (int i = 0; i < N; ++i)
    std::cout << "    void h" << i << "();\n";
  for (int i = 0; i < N; ++i)
    std::cout << "    h" << i << "();\n";

  std::cout << R"src(
  } else {
    e();
  }
}
)src";
}
```
which is still a bit too much to commit as a regression test, IMHO.

Reviewers: t.p.northover, ostannard

Reviewed By: ostannard

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69118

Change-Id: Ide5c922bcde08ff4cf635da5e52365525a997a0a