trace: Split guest_mem_before

trace: Split guest_mem_before

There is no point in encoding load/store within a bit of
the memory trace info operand.  Represent atomic operations
as a single read-modify-write tracepoint.  Use MemOpIdx
instead of inventing a form specifically for traces.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>