[SystemZ] Don't use LOAD and STORE REVERSED for volatile accesses

Unlike most -- hopefully "all other", but I'm still checking -- memory
instructions we support, LOAD REVERSED and STORE REVERSED may access
the memory location several times.  This means that they are not suitable
for volatile loads and stores.

This patch is a prerequisite for better atomic load and store support.
The same principle applies there: almost all memory instructions we
support are inherently atomic ("block concurrent"), but LOAD REVERSED
and STORE REVERSED are exceptions.

Other instructions continue to allow volatile operands.  I will add
positive "allows volatile" tests at the same time as the "allows atomic
load or store" tests.