Combinatorial processes without explicit XXX can cause circular
dependencies which causes simulators to endlessly reschedule
a loop.  Different tools have different heuristics for detecting
this and breaking up loops.  This isn't an exact science, but the
following changes seems to avoid an endless loop under Vivado XSim.

Note: There is a small behavioural change in the code:

If `ax_valid` is asserted while `ax_select >= NoMstPorts` (where `x`
is `r` or `w`), the behaviour is undefined.  In the old code this would
just not assert the `ax_ready` flag.  This is only an issue if
`ax_select` for some reason gets out of bound, which probably causes
other issues.

On the upside, the new code likely uses slightly less logic.  If the
old behaviour is required, a check to see that the select signal is
in range could be added.  Alternatively, this could be added as an
assertion.
Signed-off-by: Olof Kindgren <olof.kindgren@gmail.com>