vex/rex prefix cleanup, finally profitable to inline read_0f*_opcode

set_embedded_instructions was unnecessarily appilied to many operand
codes; this was never a correctness issue, but meant many operand
decodings took a few more instruction than necessary to do nothing.

setting all registers to `rax` is unnecessary, only the first register's
defaulting to `rax` is effectual. this allows for not using a movabs to
load initial rax state.

adjust vex decoder inlining. this will be followed up by some cleanup
for vex operand codes.