PR432809 VEX should support REX.W + POPF

PR432809 VEX should support REX.W + POPF

It seems a REX.W prefix simply explicitly sets the operant size to 8,
and so can/must be ignored as redundant. This is what we already do
for PUSH, POP and PUSHF. All instructions are described as "When in
64-bit mode, instruction defaults to 64-bit operand size and cannot
encode 32-bit operand size." in the instruction manual.

Original patch and analysis by Mike Dalessio <mike.dalessio@gmail.com>

https://bugs.kde.org/show_bug.cgi?id=432809