target/i386: Fix VSIB decode

target/i386: Fix VSIB decode

With normal SIB, index == 4 indicates no index.
With VSIB, there is no exception for VR4/VR12.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2474
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/r/20240805003130.1421051-3-richard.henderson@linaro.org
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit ac63755b20013ec6a3d2aef4538d37dc90bc3d10)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: modify the change to pre-new-decoder introduced past qemu 9.0)