[FastISel][AArch64] Make use of the zero register when possible.

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>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215591 91177308-0d34-0410-b5e6-96231b3b80d8