x86emul: support AVX512F legacy-equivalent packed int/FP conversion insns

x86emul: support AVX512F legacy-equivalent packed int/FP conversion insns

... including the two AVX512DQ forms which shared encodings, just with
EVEX.W set there.

VCVTDQ2PD, sharing its main opcode with others, needs a "manual"
override of disp8scale.

The simd_size changes for the twobyte_table[] entries are benign to
pre-existing code, but allow decode_disp8scale() to work as is here.

The at this point wrong placement of the 0xe6 case block is once again
in anticipation of further additions of case labels.

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