[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.

llvm-svn: 205091