target/i386: allow MMX instructions with CR4.OSFXSR=0

MMX state is saved/restored by FSAVE/FRSTOR so the instructions are
not illegal opcodes even if CR4.OSFXSR=0.  Make sure that validate_vex
takes into account the prefix and only checks HF_OSFXSR_MASK in the
presence of an SSE instruction.

Fixes: 20581aad ("target/i386: validate VEX prefixes via the instructions' exception classes", 2022-10-18)
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1350


Reported-by: Helge Konetzka (@hejko on gitlab.com)
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>