aarch64: support target-specific .req assembler directive

Based on the support for .req on ARM. The aarch64 variant has to keep track if
the alias register was a vector register (v0-31) or a general purpose or
VFP/Advanced SIMD ([bhsdq]0-31) register.

Patch by Janne Grunau!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212161 91177308-0d34-0410-b5e6-96231b3b80d8