Delete unused argument in AArch64MCInstLower constructor: it doesn't
use Mangler, and Mangler is in fact not even created when AArch64MCInstLower
is constructed.

This bug is reported by UBSan.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216030 91177308-0d34-0410-b5e6-96231b3b80d8