[ELF] Always write non-immediate bits for AArch64 branch instruction.

To support errata patching on AArch64 we need to be able to overwrite
an arbitrary instruction with a branch. For AArch64 it is sufficient to
always write all the bits of the branch instruction and not just the
immediate field. This is safe as the non-immediate bits of the branch
instruction are always the same.

Differential Revision: https://reviews.llvm.org/D36745

llvm-svn: 312727