x86: slightly rework handling of some register-only insns

Fold OP_MS() and OP_XS() into OP_R(), paralleling OP_M(). Use operand
names (largely) matching those in the SDM. For 128-bit-only forms use
Uxmm though, marking 256-bit forms as bad. This then allows no longer
going through vex_len_table[] for two of the insns.

Specifically _do not_ continue to mis-use v_mode.