AArch64: remove unnecessary check that RS is valid

AArch64 always demands a register-scavenger, so the pointer should never be
NULL. However, in the spirit of paranoia, we'll assert it before use just in
case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180080 91177308-0d34-0410-b5e6-96231b3b80d8