x86: improve operand reversal

x86: improve operand reversal

In quite a few cases the .s suffix or {load} / {store} prefixes did not
work as intended, or produced errors when they're supposed to be ignored
when it is not possible to carry out the request.

The change here re-purposes(?) the .s suffix to no longer mean "store"
(if that's what 's' did stand for), since the forms used in the base
templates are not consistently loads (and we unlikely want to change
that). The pseudo prefixes will now fulfill what their names say, i.e.
{load} now only ever produces a load form encoding (if available) while
{store} only ever produces a store form one (again if available). This
requires minimal test suite adjustments, while the majority of the
changes there are simply additions.