Reapply [FastISel][AArch64] Make use of the zero register when possible (r215591).

Note: This was originally reverted to track down a buildbot error. Reapply
without any modifications.

Original commit message:
This change materializes now the value "0" from the zero register.
The zero register can be folded by several instruction, so no
materialization is need at all.

Fixes <rdar://problem/17924413>.

llvm-svn: 216009