Further optimizations are possible for x64 narrow Klass decoding

<merykitty>

If we can reserve the base in the low 2G then this could be optimized further to addq(r, CompressedKlassPointers::base()) and if LogKlassAlignmentInBytes is 8 (as asserted in decode_and_move_klass_not_null below), we can shorten the whole sequence into leaq(r, Address(noreg, r, Address::times_8, CompressedKlassPointers::base())

</merykitty>