[AArch64][GlobalISel] Always fall back on tail calls with -tailcallopt

-tailcallopt requires that we perform different stack adjustments than with
sibling calls. For example, the `@caller_to0_from8` function in
test/CodeGen/AArch64/tail-call.ll requires that we adjust SP. Without
-tailcallopt, this adjustment does not happen. With it, however, it is expected.

So, to ensure that adding sibling call support doesn't break -tailcallopt,
make CallLowering always fall back on possible tail calls when -tailcallopt
is passed in.

Update test/CodeGen/AArch64/tail-call.ll with a GlobalISel line to make sure
that we don't differ from the SDAG implementation at any point.

Differential Revision: https://reviews.llvm.org/D67245

llvm-svn: 371227