target/i386: reimplement 0x0f 0x28-0x2f, add AVX

Here the code is a bit uglier due to the truncation and extension
of registers to and from 32-bit.  There is also a mistake in the
manual with respect to the size of the memory operand of CVTPS2PI
and CVTTPS2PI, reported by Ricky Zhou.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>