target/xtensa: prioritize load/store in FLIX bundles

target/xtensa: prioritize load/store in FLIX bundles

Load/store opcodes may raise MMU exceptions. Normally exceptions should
be checked in priority order before any actual operations, but since MMU
exceptions are tightly coupled with actual memory access, there's
currently no way to do it.

Approximate this behavior by executing all load, then all store, and
then all other opcodes in the FLIX bundles. Use opcode dependency
mechanism to express ordering. Mark load/store opcodes with
XTENSA_OP_{LOAD,STORE} flags. Newer libisa has classifier functions that
can tell whether opcode is a load or store, but this information is not
available in the existing overlays.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>