x86: allow to request ModR/M encoding

Several insns have a (typically shorter) non-ModR/M and a (typically
longer) ModR/M encoding. In most cases the former is used by default.
This isn't too dissimilar from register-only insns sometimes having two
encoding forms. In those cases {load} or {store} can be used to control
the encoding used. Extend this to ModR/M-less encodings which have a
ModR/M counterpart (note that BSWAP hasn't). For insn reading and
writing their (explicit) memory operand, both prefixes are honored;
otherwise only the applicable one is.

Note that for some forms of XCHG, {store} has already been performing
this function, apparently as an unnoticed side effect of adding D to
the template.