CAS code misses check for null

[Andrew Haley]

Do you think that it might be easier to have a macro which takes a dst
and does this?

void ShenandoahBarrierSet::interpreter_read_barrier(MacroAssembler* masm, Register src, Register dst) {
  Label is_null;
  __ mov(dst, src);
  __ cbz(src, is_null);
  __ ldr(dst, Address(src, BrooksPointer::BYTE_OFFSET));
  __ bind(is_null);
}

It'd make all this stuff much easier to understand.  And the
efficiency would not be very much different.

Andrew.