[ARM64] Fix 'assert("...")' to be 'assert(0 && "...")'. Otherwise, it is
no assert at all. ;] Some of these should probably be switched to
llvm_unreachable, but I didn't want to perturb the behavior in this
patch.

Found by -Wstring-conversion, which I'll try to turn on in CMake builds
at least as it is finding useful things.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205091 91177308-0d34-0410-b5e6-96231b3b80d8