AArch64: Resolve some FIXMEs in CGBuiltin left over from backend merge

Merge vrshr_n_v and vqshlu_n_v with ARM.

Remove FIXME comments for others as they can't actually be shared.

NFC.

Differential Revision: http://reviews.llvm.org/D4697


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214173 91177308-0d34-0410-b5e6-96231b3b80d8