x86emul: generalize vector length handling for AVX512/EVEX

x86emul: generalize vector length handling for AVX512/EVEX

To allow for some code sharing where possible, copy VEX.L into EVEX.LR
even for VEX (or XOP) encoded insns. Make operand size determination
use this right away, at the same time adding consistency checks for the
EVEX scalar insn cases (the non-scalar ones aren't uniform enough for
the checking to be done in a central place like this).

Note that the broadcast case is not handled here, but will be taken care
of elsewhere (in just a single place rather than at least two).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>