[AArch64] Support stackmap/patchpoint in getInstSizeInBytes

We currently return 4 for stackmaps and patchpoints, which is very optimistic
and can in rare cases cause the branch relaxation pass to fail to relax certain
branches.

This patch causes getInstSizeInBytes to return a pessimistic estimate of the
size as the number of bytes requested in the stackmap/patchpoint. In the future,
we could provide a more accurate estimate by sharing some of the logic in
AArch64::LowerSTACKMAP/PATCHPOINT.

Fixes part of https://llvm.org/bugs/show_bug.cgi?id=28750

Differential Revision: https://reviews.llvm.org/D24073

llvm-svn: 281301