x86: relax redundant REX prefix check

x86: relax redundant REX prefix check

All REX bits can be specified via individual prefixes. Redundancy should
only be reported on a per-bit basis.

Note that I originally had further checks added to the test case,
checking the effect also on PDEP. I had to strip those, because my patch
to correctly handle those
(https://sourceware.org/ml/binutils/2017-02/msg00280.html) was rejected.
I continue to think that there should not be any new prefix introduced
to handle the VEX case - whether the encoding of an insn requires VEX et
al should not be of immediate interest to the programmer.