Add new predicate volatile_mem_operand and use it for jbbssi<mode>

Fixes PR port-vax/53039: GCC/VAX hits ICE building libstdc++

GCC wants to change the label and then doesn't recognise the
new insn, it believes it doesn't satisfy the memory_operand
predicate.

It appears the memory_operand predicate doesn't accept volatile
memory accesses during the RTL generation phase.

The predicate is from rs6000 code.

from krister

(maya)