Reduce the number of nops emitted for stackmap shadows on AArch64 by counting
non-stackmap instructions up to the next branch target towards the requested
shadow.
<rdar://problem/14959522>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223156 91177308-0d34-0410-b5e6-96231b3b80d8
